Three Chinese coastguard ships sailed around the Diaoyu Islands on Friday, the State Oceanic Administration said, as China maintains its tough stance after US President Barack Obama backed Tokyo in its territorial dispute with Beijing.

According to China News Service, it was the Chinese vessels' third such patrol since Obama reasserted on April 24 that Washington would defend Japan under a bilateral military treaty if China initiated an attack in the dispute over the islands.

Beijing has dismissed Obama's remarks, saying the islands are "China's inherent territory".

Foreign Affairs University Japan studies researcher Zhou Yongsheng said the Chinese coastguard's regular patrols have exposed the awkward situation Washington and Tokyo face over the islands.

"Chinese ships and aircraft have maintained regular patrols in the area since Japan illegally 'nationalized' the islands in September 2012," Zhou said.

"The Diaoyu Islands are no longer controlled solely by Japan, and Japan is incapable of stopping China from patrolling. Still, the United States ignored the facts and continued showing partiality for Japan. Obama's remarks appear meaningless in the face of Beijing's nonstop patrols."

Friday's patrols came one day after Japan announced it will stage large-scale landing drills in the East China Sea from May 10 to 27.

The plan was made public hours after the Xinhua News Agency quoted the Ministry of National Defense as saying the Chinese and Russian navies will conduct joint exercises "off Shanghai" in late May. It is speculated the exercises will take place in waters northwest of the Diaoyu Islands.

The Kyodo news agency said China may want to use the joint drills to demonstrate its coordination with Russia in the East China Sea and keep Japan and the US in check.

It also said Japan's landing drills "are seen as a move to keep China in check".

Zhou said Washington's and Tokyo's economic sanctions on Moscow over Crimea have pushed Russia closer to China.

Japan and Russia also have a long-running territorial dispute centered on four islands off Japan's northern coast.

"Obama's partisan remarks over the Diaoyu Islands have also pushed China to cooperate with Russia to maintain balance in regional geopolitics," Zhou said.

"The two countries have said their drill is a regular arrangement that does not target a third country. But it is not a bad thing if Japan and the international society want to link the event to Tokyo.

"Washington's partiality in favor of Japan has accelerated regional turbulence. The US and Japan should be aware they are responsible for that."

AFP contributed to this story.

[email protected]