The United States has no plans this year for tripartite ballistic missile defense exercises with South Korea and Japan despite Washington's continued push for bolstering three-way military cooperation with the key regional allies, according to a Pentagon source.

"I do not believe we have any trilat missile exercise scheduled this year," the source told Yonhap News Agency.

The three countries had the Pacific Dragon exercise last year, in which they successfully tracked two near-simultaneous launches of target ballistic missiles.

It was aimed at honing air defense capabilities against North Korea's ballistic missile threats. In April, the nuclear-armed communist nation successfully launched a space rocket, which was viewed by the U.S. as a test of intercontinental ballistic missile.

Pentagon officials said they were satisfied with the results of the Pacific Dragon training held off the coast of the Korean Peninsula and hoped for follow-up drills.

The U.S. considers boosting trilateral cooperation with the regional allies on regional missile defense as an option, a senior defense department official said earlier.

The Pentagon's decision to skip a trilateral missile defense exercise this year may be attributable to the North's recent peace offensive, another source said.

Political and diplomatic ties between Seoul and Tokyo also show no signs of improving.

For now, the Pentagon is apparently focusing on preparing for a possible strike on Syria.

Meanwhile, the U.S. went ahead with its own missile defense test this week.

The Pentagon announced Tuesday its troops "successfully conducted a complex missile defense flight test, resulting in the intercept of two medium-range ballistic missile targets."

The flight test in the western Pacific was "planned more than a year ago, and is not in any way connected to events in the Middle East," it added.

U.S. ballistic missile defense system programs have completed 62 successful hit-to-kill intercepts in 78 flight test attempts since 2001, according to the Pentagon. (Yonhap News)