By Tim Kelly and Phil Stewart
TOKYO/WASHINGTON (Reuters) - 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.
The P-8As, built by Boeing Co based on its 737 passenger plane, were been built to replace the aging propeller-powered Lockheed Martin P-3 Orion patrol aircraft, which have been in service for 50 years.
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.
One U.S. official, speaking on condition of anonymity, acknowledged the deployment came at a moment of region tensions. The official said the timing was a coincidence, but one that would bolster the U.S. military's efforts to monitor the maritime environment in the region.
That includes the area around the disputed islands, known to the Japanese as the Senkakus and to the Chinese as the Daioyu, the official said.
Washington takes no position on the sovereignty of the islands. However, it recognizes Tokyo's administrative control and says the U.S.-Japan security pact applies to them, a stance that could drag the United States into a military conflict it would prefer to avoid.
(Editing by Nick Macfie and Christopher Wilson)