The US Navy has deployed a pair of sophisticated surveillance aircraft to Japan, officials said, with four more due to arrive this week amid rising tension over China's territorial claims in the region.
Two P-8A Poseidon jets arrived at Kadena air base in Okinawa on Sunday, in a move that was planned before Beijing declared an air defence identification zone last month covering disputed islands in the East China Sea, a navy official said.
"This was scheduled for a long time," said the official, who spoke on condition of anonymity. "It's a rotational deployment."
The other four were expected to arrive there later this week, a navy spokeswoman 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 other vessels.
"The P-8A is the most advanced long-range anti-submarine and anti-surface warfare aircraft in the world," the Pentagon said.