SEOUL, South Korea -- At a time of growing concern about North Korea's missile program, South Korea, Japan, and the United States will hold their first-ever joint anti-missile exercise next month, South Korea's Defense Ministry told CNN Monday.
"It will involve one Aegis-level vessel from each country," said the ministry, referring to warships that can shoot down enemy ballistic missiles.
The three countries will practice "detecting and tracing a hypothetical North Korean missile," said a ministry official. But the drill will not involve firing an actual missile to practice interception. Instead, the drill will be focused on the exchange of information among the units involved.
Pentagon spokesman Maj. Jamie Davis said he could not comment on specifics, but the exercise would be held on the sidelines of the Rim of the Pacific Exercise (RIMPAC) event in June. Headquartered in Hawaii, RIMPAC is described by the U.S. Navy as "the world's largest international maritime warfare exercise."
The exercises were developed within the context of an information-sharing agreement signed by the three countries in December of 2014, Davis said.
"That's phenomenal. Almost unprecedented," said retired Army Gen. Spider Marks, a former top American military intelligence officer in South Korea.
Japan and South Korea have only limited military cooperation, due to decades of tensions dating back to World War II. But Marks said the growing threat posed by North Korea has apparently pushed them to work together.
"They are working now hand in hand because they have a sense of a joint threat," he said.
North Korea has tested several weapons systems so far this year: a nuclear device, a three-stage rocket, a submarine-launched missile, a mobile-launched missile, and a new long-range rocket engine. It has also claimed to have miniaturized a nuclear device to fit onto a long-range missile.
Next month's joint missile defense exercises "signal the seriousness with which all of these countries see the threat from North Korea," said Thomas Karako at the Center for Strategic and International Studies.
While next month's drill will only involve sharing tracking data, and not shooting down a missile, it could lead to more-sophisticated tests in the future, Karako said. "First making sure that the radars on the ships can talk together, and share information, that's an initial first step," he said.
But Karako warned that the system is most useful for intercepting one or two missiles, not a whole salvo. Moreover, he and Marks pointed out, the Aegis system is not intended to defend against the threat of short-range missiles on land, or artillery. Those two types of North Korean weapons pose a more immediate threat to South Korea, and to the 28,500 American forces based there.
"North Korea has always had an incredible artillery capability and missile capability, and with very little warning, could launch an attack," Marks said.
The Aegis is an American missile defense system primarily deployed at sea intended to intercept short- and medium-range ballistic missiles, according to the Defense Department's Missile Defense Agency. It is also deployed on several Japanese ships.
China and Russia have expressed opposition to South Korean cooperation with the U.S. on missile defense systems, seeing it as disruptive to the regional military balance. In particular, the two countries last month criticized an American proposal to deploy the land-based Terminal High Altitude Area Defense system, or THAAD, in South Korea.
So far, that system has not been deployed in South Korea.
CNN's Michael Pearson and Brian Todd contributed to this report.
By KJ Kwon and Dugald McConnell