The United States and Japan are keeping a close eye on North Korea as the communist nation prepares to fire what neighbouring countries suspect is a long-range missile capable of hitting North America.

North Korea has said it will be launching a communications satellite, but Japan, South Korea and the United States believe Pyongyang's account is a cover for a missile launch, since missiles and satellites have very similar launch delivery systems.

Analysts say satellite imagery shows heightened activity at a launch pad in North Korea's northeast.

U.S. military officials at Misawa Air Base, which is on Japan's northern fringe just across the Sea of Japan from North Korea, said they are closely watching the situation but refused to provide any details about the possible timing of the launch.

"U.S. Army forces in Japan are always vigilant and more than capable of defending Japan from any threat, including ballistic missiles, and as always we are working closely with our Japanese allies," Maj. James Crawford, a spokesman for the U.S. Army in Japan, said Tuesday.

Japan mulls sending missile interceptors: report

Japan is considering the deployment of two destroyers equipped with radar and missile interceptors to the sea of Japan, reported the Kyodo news agency Tuesday, citing an unnamed defence official.

A Japanese Defence ministry spokeswoman declined comment.

It is natural for Tokyo to "respond to any situations" when asked whether the government plans to shoot down a North Korean missile, Japan's Defence Minister Yasukazu Hamada said.

Domestic media reported earlier in the week that Japanese Prime Minister Taro Aso had indicated that his country should be able to use missile defence systems — even if North Korea claims it is simply launching a satellite.

Japan, in conjunction with the United States, has worked to set up a missile defence system consisting of interceptors located at bases on the mainland as well as on ships at sea.

Japan has long been anxious about missile activity from North Korea, which in 1998 launched a long-range ballistic missile over its main island and well into the Pacific, almost reaching Alaska.

It's already been a tense week as talks with Pyongyang, Seoul and the U.S.-led UN command began amid heightened tensions over the suspected missile launch. North Korea on Monday warned the U.S. and South Korea to hold off on a joint annual military drill in the south.

Pyongyang sees the exercise as preparations for an attack, an assertion that the U.S. and South Korea rejected.

With files from the Associated Press