Japan put its military on alert on Monday for a possible North Korean ballistic missile firing, while South Korea also said it had detected evidence of launch preparations, officials from Japan and South Korea said.
Tension in the region has been high since North Korea conducted its fourth nuclear test in January and followed that with a satellite launch and test launches of various missiles.
Japan ordered naval destroyers and Patriot anti-ballistic missile batteries to be ready to shoot down any projectile heading for the country, state broadcaster NHK said.
A Japanese official, who declined to be identified as he is not authorized to speak to the media, confirmed the order. A spokesmen for Japan's defense ministry declined to comment.
The missile tubes on a Patriot missile battery on the grounds of Japan's Ministry of Defense were elevated to a firing position.
The South Korean defense official declined to comment on what type of missile might be launched, but South Korea's Yonhap News Agency said officials believe it would be an intermediate-range Musudan missile.
"We've detected a sign and are tracking that. We are fully prepared," said the South Korean official, who also declined to be identified.
A Pentagon spokesman, U.S. Navy Commander Gary Ross, said: "We are closely monitoring the situation on the Korean Peninsula in coordination with our regional allies. We urge North Korea to refrain from provocative actions that aggravate tensions and instead focus on fulfilling its international obligations and commitments."
Ross said he would not discuss U.S. intelligence assessments. The White House declined to comment.
North Korea tried unsuccessfully to test launch the Musudan three times in April, according to U.S. and South Korean officials.
Japan has put its anti-ballistic missile forces on alert at least twice this year after detecting signs of launches by North Korea.
North Korea's nuclear and missile tests this year triggered new U.N. sanctions. But it seems determined to press ahead with its weapons programs, despite the sanctions and the disapproval of its sole main ally, China.
Last Friday, leaders of the Group of Seven industrialized nations, including Japanese Prime Minister Shinzo Abe and U.S. President Barack Obama, met in Japan and demanded that North Korea comply with a U.N. Security Council resolution to stop all nuclear and missile tests and refrain from provocative action.
On the same day, North Korea threatened to retaliate against South Korea after it fired what it said were warning shots when boats from the North crossed the disputed sea border off the west coast of the Korean peninsula.
Japan has advanced Aegis vessels in the Sea of Japan that are able to track multiple targets and are armed with SM-3 missiles designed to destroy incoming warheads in space before they re-enter the atmosphere.
Patriot PAC-3 missile batteries, designed to hit warheads near the ground, are deployed around Tokyo and other sites as a second and final line of defense.