The US is considering deploying Patriot interceptor missiles at its bases in Japan, amid fears North Korea is about to test-launch a long-range missile.
N Korea has not launched long-range missiles since 1998
The deployment would mark the first time the US has deployed surface-to-air missiles in Japan.
As part of ongoing diplomatic efforts to avert the North's test launch, South Korea's Foreign Minister Ban Ki-moon was due to visit China on Monday.
He said he would ask Beijing to play a constructive role to resolve the issue.
China is one of North Korea's few remaining allies, and still has some influence in Pyongyang.
According to US intelligence reports, North Korea may be preparing to launch a Taepodong-2 missile, which has a range of up to 6,000km (3,730 miles), putting parts of the US within striking distance.
Some analysts think the North sees a threatened launch as a way to strengthen its diplomatic hand.
The US and North Korea's neighbours have already issued several warnings to Pyongyang not to go ahead with the launch.
Concerns about the potential launch have brought regional defence systems into the spotlight.
Japan has agreed to US plans to place Patriot Advanced Capability-3 missiles on some of its bases in Japan for the first time, a Defence Agency official said on Monday.
These PAC-3 missiles are designed to intercept ballistic or missiles, as well as aircraft.
The missiles would be deployed on the southern island of Okinawa, according to a report in Japan's largest newspaper Yomiuri Shimbun.
An additional 500-600 troops would also be deployed, the newspaper said.