Japan's defence minister vowed Sunday to defend the country's territory as three Chinese government ships entered disputed waters off Tokyo-controlled islands in the East China Sea, the first such incident this year.

The Chinese coastguard vessels sailed into the 12-nautical-mile territorial waters at about 8:30 am (2330 GMT Saturday) off one of the Senkaku islands, which China also claims and calls the Diaoyus, Japan's coastguard said. They left less than two hours later.

"We can never overlook repeated incursions into territorial waters," Japanese Defence Minister Itsunori Onodera told reporters.

"We need to make diplomatic efforts on one hand. We also want to firmly defend our country's territorial sea and land with the Self-Defence Forces cooperating with the Coast Guard," he added.

Chinese state-owned ships and aircraft have approached the Senkakus on and off to demonstrate Beijing's territorial claims, especially after Japan nationalised some of the islands in September 2012.

It was the first time Chinese ships had been spotted since December 29 when three coastguard ships entered the zone and stayed for around three hours, the Japan Coast Guard said.

Japanese coastguard patrol boats have tried to chase Chinese vessels away, fuelling tensions which some fear could spiral out of control into an armed clash.

Japan's conservative Prime Minister Shinzo Abe has vowed no compromise on the sovereignty of the islands and recently announced a boost in military spending to beef-up the nation's defence.