The U.S. Navy's first two advanced P-8A Poseidon patrol aircraft have arrived in Japan, U.S. military officials said on Monday, helping to upgrade America's ability to hunt submarines and other vessels in seas close to China as tension in the region mounts.

The initial deployment - another four of the aircraft are due to arrive in the coming days - was planned before China last month established an air defense identification zone covering islands controlled by Japan and claimed by Beijing.

The Pentagon says it is routinely flying operations in the region, including in China's newly declared air defense zone, without informing Beijing ahead of time.

One U.S. defense official, speaking on condition of anonymity, told Reuters these routine operations include surveillance flights.

The deployment of the P-8As came before U.S. Vice President Joe Biden left for Asia this week, where he is seeking to strike a balance between calming military tensions with China and supporting Japan as it wrangles with Beijing over the islands.

Equipped with the latest radar equipment and armed with torpedoes and anti-ship missiles, the P-8A is able to fly further and stay out on a mission longer than the P-3.

"The P-8A is the most advanced long-range anti-submarine and anti-surface warfare aircraft in the world," the Pentagon said.

Lieutenant Colonel Jeffrey Pool, a Pentagon spokesman, said the first two P-8As arrived in Japan on Sunday. The other four were expected to arrive there later this week, a Navy spokeswoman said.

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