TOKYO (AFP) ― Three Chinese coastguard ships sailed through disputed waters off Tokyo-controlled islands in the East China Sea on Sunday, the latest such incident in a bitter territorial row between the Asian giants.

The Chinese vessels entered the 12-nautical-mile territorial waters off one of the Senkakus, which China claims and calls the Diaoyus, at around 9 a.m. and left the area around two hours later, the Japanese coastguard said.

It was the fifth time this year that state-owned Chinese ships entered the zone and the first foray in six days.

China’s State Oceanic Administration said on its website: “On Feb. 23, China’s coastguard ships 2151, 2113 and 2102 patrolled together into territorial waters surrounding China’s Diaoyu islands.”

Chinese vessels and aircraft have regularly approached the East China Sea archipelago ― thought to harbor vast natural resources ― after Japan nationalized some of the islands in September 2012, setting off the latest spate of incidents in a long-running territorial dispute.

Fears of conflict rose in November when China imposed an air defense identification zone over the East China Sea and said it required notification from planes crossing the area.

Washington was angered by the move, saying it could lead to confusion in the skies.

U.S. Secretary of State John Kerry has warned Beijing against unilateral moves to set up a similar air defense zone over the South China Sea, where the Philippines in particular has voiced worries about China’s maritime claims.

Earlier this month, the top U.S. diplomat said the United States would defend Japan against attack including over the disputed islands in the East China Sea.