North Korea released 16 Chinese fishermen and their boat Tuesday, Beijing said, demanding an explanation after the kidnapping by armed attackers heightened strains between the neighbours.

"The DPRK side released the fishing boat and all the fishermen were safe and healthy," foreign ministry spokesman Hong Lei told a regular press briefing, using the North's official name.

The vessel's owner had not paid a ransom, he added, calling on Pyongyang to launch "a full investigation into the incident and make an explanation to us and take effective measures to prevent the reoccurrence of such incidents".

The seizure is the latest strain in the relationship.

Owner Yu Xuejun told AFP that armed North Koreans whom he said were probably from Pyongyang's military detained the boat in waters between the two countries on May 6 and demanded 600,000 yuan ($98,000) for the men's release.

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

North Korea has for years done most of its banking through China, but with the imposition of stronger UN sanctions after the nuclear test, Beijing has come under greater pressure to tighten its control on Pyongyang's financial flows.

The state-owned Bank of China in early May shut the account of a North Korean bank accused by the United States of supporting the atomic programme.

Reports said the boat's captors had asked Yu to pay the ransom into a bank account in the northeastern Chinese city of Dandong, a major hub for trade between China and the North.

The detention caused outrage online in China, with Internet users calling on Beijing to take a tough stance against Pyongyang, and accusing authorities of not trying hard enough to secure the men's release.

In an editorial Tuesday before the release was announced, the state-run Global Times, which often reflects nationalist opinion, said Beijing 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, adding: "If we don't set rules for North Korea, our whole government's image may be seen as being too weak."

The incident came a year after the return of 29 Chinese 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 to try to resolve the case, Xinhua reported at the time.

"There is no clear demarcation of the sea border between China and North Korea," the Global Times quoted Lu Chao, a Chinese maritime researcher, as saying.

"Whenever North Korean coastal troops lack money, they cross the line and detain Chinese vessels to extort money. And most ship owners choose to pay the ransom if the amount is not too high," he said.