BEIJING (AFP) - North Korea has released 16 Chinese fishermen and their boat, Chinese state-run media said on Tuesday, after reports that armed assailants had taken the sailors hostage and demanded a ransom.

"All the fishermen with the boat are safe on their way back," China's Xinhua news agency said, citing a Chinese embassy official in Pyongyang it said had heard the news from the shipowner.

Xinhua said the fishermen had been detained "by the Democratic People's Republic of Korea side", using the North's official name, and without giving further details.

The vessel's owner Yu Xuejun told AFP that armed North Koreans seized the boat in waters between the two countries on May 6 and demanded 600,000 yuan (S$122,600) for the men's release, adding they were probably from Pyongyang's military.

The incident is the latest strain in the relationship between the neighbours.

Beijing is Pyongyang's sole major ally and its key provider of aid and trade, but China said it "firmly opposed" the nuclear-armed North's atomic test in February.

China has stayed in "close communication" with North Korea over the incident, foreign ministry spokesman Hong Lei said at a regular news briefing on Monday.

The detention caused outrage online in China, with Internet users accusing the authorities of not making sufficient efforts to secure the men's release.

In an editorial Tuesday before the release was announced, the state-run Global Times said China should "should let the North Korean side know we are angry".

"If North Korea continues to go rogue, China should take actions to push it toward a more measured response," it said. "If we don't set rules for North Korea, our whole government's image may be seen as being too weak to deal with maritime issues."

The incident came a year after the return of 29 fishermen also kidnapped by unidentified North Koreans who had demanded a 1.2 million yuan ransom.

Those sailors were returned without ransom after the foreign ministry said it had contacted North Korea in an effort to resolve the case, Xinhua reported at the time.