WASHINGTON (CNN) -- U.S. Navy ships capable of shooting down ballistic missiles are being moved to the Sea of Japan, a Navy spokesman said.

The move came as North Korea was preparing for an expected rocket launch next month.

Later Thursday, Japan announced it was ready to fire on the rocket if any part of it enters Japanese airspace.

Defense Minister Yasukazu Hamada's order was to destroy debris from the North Korean rocket if its launch fails and fragments fall in Japanese territory, said defense ministry spokesman Yuichi Akiyoshi.

The U.S. ships, with powerful Aegis radar that can track ballistic missile launches, were on regularly planned deployments but were, "prepared to track a launch or more, if afforded," according to a U.S. Navy official who could not be named due to the sensitivity of the information.

The United States generally has a number of Aegis-capable ships in the Sea of Japan because of the threat posed by North Korea to launch missiles. The ships monitor the region and are designed to track and, if need be, shoot down ballistic missiles. See a satellite image of a North Korean launch site »

North Korea says it will launch a commercial satellite on top of a rocket sometime between April 4 and April 8. But Western governments fear the North Koreans will actually put a long-range missile on top of the rocket.

If North Korea launches, the Obama administration may have as little as five minutes to decide whether it is a threat and, if necessary, try to shoot it down.

The USS Hopper, a destroyer with the Aegis radar system aboard, was scheduled for a port call in Japan in coming days. But the port call was canceled and the ship will remain in the Sea of Japan ahead of the launch, the official said.

Two other U.S. Navy Aegis-capable destroyers, the USS Chaffee and USS McCain, were leaving the port in Sasebo, Japan, and were heading to South Korea for a ceremony in coming days, according to the U.S. Navy official with direct knowledge of the operations.

The U.S. Navy just wrapped up military exercises with the South Korean military, bringing a number of its ships into the region.

-- CNN's Mike Mount at the Pentagon contributed to this report.

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