Seoul, South Korea ? The United States and Japan agreed Friday to strengthen cooperation on missile defense amid concerns of a possible long-range rocket launch by North Korea.

The accord came as U.S. forces ended five days of Pacific war games – the largest in the region since the Vietnam War. The exercise brought together three aircraft carriers along with 22,000 troops and 280 warplanes off the island of Guam.

In Tokyo, Japanese Foreign Minister Taro Aso and U.S. Ambassador Thomas Schieffer signed documents about cooperation on ballistic missile defense development. Japan’s Defense Agency also said a high-resolution radar that can detect a ballistic missile had been deployed at a base in northern Japan.

North Korea has made recent moves that would enable it to launch a long-range missile, U.S. and Asian officials have said. Intelligence reports say fuel tanks have been seen around a missile at North Korea’s launch site on its northeastern coast, but officials say it’s difficult to determine if the rocket actually is being fueled by looking at satellite photos.

The North has said it is willing to talk with the U.S. about its missile concerns, repeating its long-held desire for direct meetings with the Americans. But the U.S. has refused, and insists it will meet the North only in six-nation talks aimed at ridding Pyongyang of its nuclear weapons program.