The US Navy said Thursday it had deployed two warships in waters off Japan ahead of North Korea's planned rocket launch early next month.
Two destroyers fitted with Aegis technology to track and destroy missiles left port on Wednesday, US Navy public relations officer Charles Howard told AFP.
"I would say we are ready for any contingencies," he said.
Pyongyang says it will launch a communications satellite between April 4 and 8. The United States and its Asian allies suspect the launch is a test of a long-range ballistic missile that could reach North America.
Tensions have risen after US officials said overnight that North Korea had moved a Taepodong-2 missile to the Musudan-ri launch site on the country's northeast coast.
The US Navy spokesman said two Aegis-equipped destroyers -- the USS McCain and USS Chafee -- left Sasebo port in southwestern Japan where a third, the USS Curtis Wilbur, remained docked.
The USS Stethem, another Aegis-equipped destroyer, was set to leave northern Aomori port Thursday after a port call of several days, a US Navy spokeswoman said, without specifying its destination.
The Japanese navy's two Aegis-equipped destroyers Kongo and Chokai are on stand-by at Sasebo, a defence ministry spokeswoman said.
A South Korean Aegis-equipped warship is also likely to be deployed in waters near Japan, public broadcaster TBS reported, without citing sources.
The North's missiles have alarmed Japan since a Taepodong-1 overflew its territory in 1998. The first test of a longer-range Taepodong-2 in 2006 failed after 40 seconds.
Japan's security council will meet this week to prepare for the shooting down of any rocket or debris which threatens to strike its territory.
General Ryoichi Oriki, chief of staff of the Japanese Self Defence Forces, told a media briefing: "We will take all measures possible to ensure safety once the government order is issued."
Asked when he would deploy surface-to-air Patriot missiles and the destroyers, he said: "We want to deploy swiftly after the order."