TOKYO (AFP) - Two Chinese coastguard ships sailed into waters around disputed islands in the East China Sea on Saturday, the Japanese coastguard said, two days after US President Barack Obama delared his support for Japan.

The vessels entered 22km into Japan's territorial waters off one of the Senkaku islands, which China also claims and calls the Diaoyus, around noon local time, the coastguard said.

It was the first such move since Mr Obama announced on Thursday that Washington would defend Japan, under the bilateral military alliance, if China initiates an attack in the tense territorial dispute. China has already dismissed Mr Obama's position, saying that the islands are "China's inherent territory."

Chinese ships last entered the area on April 12, according to the Japanese coastguard. Chinese vessels and aircraft regularly approach the East China Sea archipelago - thought to harbour vast natural resources - after Japan nationalised some of the islands in September 2012, setting off the latest spate of incidents in a long-running territorial dispute.

Relations between Tokyo and Beijing have fallen to their lowest point for years. Some observers warn they might come to blows over the islands, where ships from both sides lurk to press claims for ownership.