DONETSK, Ukraine (CNN) -- On the ground, recovery experts began the grisly task of collecting remains of the 298 people killed two days earlier when a Malaysia Airlines jet exploded over the war zone of eastern Ukraine.
Across diplomatic channels, pressure mounted on Russian President Vladimir Putin to rein in pro-Russia rebels, backed by Moscow, who are accused of firing the missile that downed the commercial plane and of hindering access to its debris field.
Saturday brought a shift from the initial shock over the air disaster to the painstaking forensic and diplomatic challenges of figuring out what happened and how much the tragedy will escalate the Ukrainian conflict.
Masked gunmen at crash site
At the crash site, armed gunmen -- some of them masked -- kept close watch over a team of observers from the Organization for Security and Co-operation in Europe who gained access for a second day. Artillery fire reverberated in the distance.
Michael Bociurkiw, a spokesman for the OSCE group, told CNN that no security perimeter had been established and no one appeared to be in charge. While he said his monitors had better access than on Friday, observing about 75 body bags collected by civilian emergency workers, he described the situation as far from ideal for such a huge crime scene.
Meanwhile, international pressure increased on Russia to exert its influence over separatist rebels it supports who are fighting the Ukrainian government in the region.
European leaders including German Chancellor Angela Merkel and British Prime Minister David Cameron called on Putin to ensure free access to the crash site for a proper investigation.
Dutch Prime Minister Mark Rutte, whose citizens comprised almost two-thirds of the victims on the flight from Amsterdam to Kuala Lumpur, called images of people rummaging through the debris and belongings of victims "downright disgusting."
'A very intense conversation'
Rutte told reporters of what he called "a very intense conversation" with Putin on Saturday in which he told the Russian leader "the opportunity expires to show the world that he is serious about helping."
Cameron, whose government summoned the Russian ambassador to urge more cooperation on the matter, said the European Union needs to reconsider its approach to Russia in light of evidence that the rebels fired the fatal missile.
Meanwhile, U.S. Secretary of State John Kerry spoke with his Russian counterpart, Foreign Minister Sergey Lavrov, to urge Moscow to get the rebels to stop fighting and talk peace, and also provide full access to the crash site.
The United States has said a surface-to-air missile fired from the rebel territory took down the Malaysia Airlines Boeing 777 with citizens from more than 10 nations aboard.
Russia has denied any involvement, and Putin said Ukraine's military campaign against the separatists was to blame. He also has called for a "thorough and objective investigation" of the crash.
Since the crash, the Ukrainian government and rebels have traded bitter accusations over who was responsible and what has been done since.
Vitaly Nayda, counterintellligence chief for Ukraine's Security Service, told reporters in Kiev that a Russian-made Buk M1 missile system had shot down the Malaysian airline.
He claimed that three Buk surface-to-air antiaircraft missile systems had crossed from Russia to Ukraine prior to the downing of Flight MH17, accompanied by Russian nationals who, he said, were the ones operating the sophisticated weaponry. All three Buk missile systems are no longer in Ukrainian territory, according to Nayda.
Prime Minister Arseniy Yatsenyuk, in an interview with CNN on Saturday, also suggested that whoever operated the missile system received expert training.
"This is not the Russian-led drunk terrorist who pressed the button," he said. "This is someone well-trained. Someone who knows how this machine works. Someone who has experience."
'Crime against humanity'
Ukraine and the international community "will find out all responsible for this international crime, and those who supported them, because this is (a) crime against humanity, and the building of (the) International Criminal Court is very big," Yatsenyuk added.
Even the local head of the rebels conceded for the first time Saturday that the plane got shot down. But Alexander Borodai reiterated that his forces did not do it. He told reporters the rebels lacked the firepower to hit an airplane so high up.
According to government officials, the rebels also removed debris and 38 bodies from the scene as part of an attempt to cover up what happened, and money, jewelry and other items had been looted from the dead. They urged relatives to cancel the credit cards of victims.
Borodai, the rebel leader who calls himself the prime minister of the self-styled Donetsk People's Republic, denied that his forces removed any bodies.
"There is even a house where a body fell, the landlord asked us to remove, and we haven't because we are not allowed to move anything," he said.
A CNN crew at the scene Saturday said it did not see any signs of looting or the rebels rummaging through items at the crash site.
Access an issue
The fields where the plane came down Thursday, near the town of Torez in the Donetsk region, are in a volatile rebel-controlled area, making access to the scattered debris, bodies and body parts difficult.
Ukrainian officials blamed the security issues on factionalism and lack of internal communication among the pro-Russian rebels.
Some rebel groups in the area have agreed to give OSCE experts access to the wreckage, while others have not, a spokesperson for Donetsk Gov. Oleksandr Omelchenko said. He said the conditions make it impossible to know whether all the armed rebels have left an area.
This is consistent with what CNN journalists have seen at the debris field. Earlier Saturday, a rebel commander on the ground gave a group of CNN journalists permission to approach the wreckage, but within 30 minutes warning shots were heard and the journalists were told to leave.
One key issue for investigators is the location of the plane's flight data recorders, which may hold crucial data.
The Ukrainian government said Friday that the so-called black boxes are still in Ukrainian territory but didn't clarify whether they were in Ukraine's possession.
Bociurkiw of the OSCE said no one at the crash site was able to tell his people where the recorders might be.
Malaysian investigators also touched down in Kiev on Saturday to try to get the bottom of what happened to the jetliner.
But Malaysia's official news agency Bernama said they were still negotiating with pro-Russian rebels over access for their 131-member team.
Malaysian Transport Minister Liow Tiong Lai said Saturday in Kuala Lumpur that Malaysia was "deeply concerned that the crash site has not yet been properly secured."
"There are indications that vital evidence has not been preserved in place," he said.
In an indication of the volatility of the region, at least five Ukrainian soldiers were killed and 20 wounded in the past 24 hours in clashes with pro-Russian separatists only 100 kilometers from the debris fields, Lt. Col. Vladislav Seleznyov told CNN on Saturday.
Fighting is taking place around the Luhansk airport and the Metalist neighborhood of Luhansk, said Seleznyov, a Ukrainian military spokesman. The pro-Russian separatists are firing with heavy artillery, mortars and Grad rockets, he said.
An international tragedy
The full list of the passengers was released Saturday. According to a final breakdown from Malaysia Airlines, 193 of those killed were from the Netherlands, including one who had dual U.S.-Dutch citizenship.
There were also 43 victims from Malaysia, including the plane's 15 crew; 27 from Australia; 12 from Indonesia; 10 from the United Kingdom, including one who had dual UK-South African citizenship; four each from Germany and Belgium; three from the Philippines and one each from Canada and New Zealand.
Eighty of the victims were children, the United Nations said.
In the Netherlands, dozens of police officers are now visiting all the families of the victims. They will gather specific information that will help identify the victims, such as DNA samples, details of tattoos and dental records, the Dutch police said. A Dutch forensics team has already arrived in Ukraine.
The FBI is sending two investigators to work on the case, a U.S. law enforcement official said, but the Ukraine government will be in charge of the investigation.
Australia is sending six foreign affairs officers to Kiev to assist in the investigation, the country's Prime Minister Tony Abbott said Saturday.
The Kremlin has criticized Abbott over his harsh words on possible Russian involvement in the tragedy. He repeated them Saturday.
"Australia takes a very dim view of countries which facilitate killing of Australians, as you'd expect us to. We take a very, very dim view of this and the idea that Russia can wash its hands of responsibility, because this happened in Ukrainian airspace, just does not stand serious scrutiny," Abbott said.
Obama's focus on Russia
U.S. President Barack Obama also said Russia likely bears some of the responsibility, noting rebel fighters couldn't have operated the surface-to-air missile "without sophisticated equipment and sophisticated training, and that is coming from Russia."
A day before MH17 came down, Obama announced expanded sanctions against a number of major Russian companies in response to Russia's actions in Ukraine.
In an apparently retaliatory move, Russian Foreign Ministry spokesman Alexander Lukashevich said Saturday that Russia had added the names of a number of American citizens to a list that bans them from entering Russia.
Tensions have been high between Ukraine and Russia since street protests forced former pro-Moscow President Viktor Yanukovych from power in February. Russia subsequently annexed Ukraine's southeastern Crimea region, and a pro-Russian separatist rebellion has been raging in Ukraine's eastern Luhansk and Donetsk regions.
Ukraine has accused Russia of allowing weapons and military equipment, including tanks, to cross the border illegally into the hands of pro-Russian rebels.
By Tom Cohen, Laura Smith-Spark and Phil Black
CNN's Phil Black reported from Donetsk and Laura Smith-Spark reported and wrote from London and Tom Cohen reported and wrote from Washington. CNN's Nic Robertson, Ivan Watson, Barbara Starr, Elise Labott, Pam Brown, Jim Sciutto, Brian Walker, Ralph Ellis, Paul Ferguson and Antonia Mortensen also contributed to this report, as did journalists Victoria Butenko and Azad Safarov.