Two P-8A Poseidon patrol jets departed Jacksonville, Florida on Friday and arrived later at Kadena air base in Okinawa, in a move that was planned before Beijing declared an air defense zone last month covering disputed islands in the East China Sea, a Navy official told AFP.

"This was scheduled for a long time," said the official, who spoke on condition of anonymity.

"It's a rotational deployment."

Four more Poseidon aircraft are due to deploy at Okinawa later this month, the official said.

The assignment to Japan marks the first mission for the new plane, which is replacing the propeller-driven P-3 Orion aircraft that dates back to the 1960s.

The P-8A planes, converted Boeing 737s equipped with advanced radar and anti-ship missiles, are designed to hunt submarines and track other vessels at sea.

The deployment came as US Vice President Joe Biden set out on a trip Sunday to Asia which will include a visit to Beijing, where he will discuss regional tensions that have spiked since Beijing's declaration.

Senior US officials said Biden would convey Washington's "concerns" about China's air defense zone and seek clarity regarding its intentions.

On November 23, China announced an expanded air defense identification zone and said aircraft  would have to submit flight plans before entering the area, home to disputed islands in the East China Sea.

China and Japan have been locked in a mounting territorial argument over the island chain, known as Senkaku in Japanese and Diaoyu in Chinese.

The United States has said it will not alter its military operations in the area despite China's declared zone and sent two B-52 bombers through the area last week.

The US State Department, however, has said American commercial airlines should observe China's demand to be given advance notice of aircraft entering the zone.

Although taking no position on the sovereignty dispute, Washington says it would uphold its defense treaty with Japan in the event of a conflict.

NJF/NJF