Japan condemned Monday's incursion as a "serious violation" of its sovereignty, saying Beijing was becoming "increasingly active".

Washington's closest ally in the region, Japan said it scrambled fighter jets after the two-minute incursion by the Y-9 surveillance aircraft on Monday off the Danjo Islands in the East China Sea.

Asked about Japan's claims, foreign ministry spokesman Lin Jian said "the relevant Chinese departments are in the process of gathering information and verifying the situation".

"The two sides have maintained communication through existing working channels," he said.

"I would also like to emphasise here that China has no intention of intruding into the territorial airspace of any country," Lin said.

The uninhabited Danjo Islands are a group of small islets located in the East China Sea off Japan's southern Nagasaki region and are not disputed territory.

Japan, staunchly pacifist for decades, has ramped up defence spending with US encouragement, moving to acquire counter-strike capabilities and easing rules on arms exports.