A number of Chinese fishermen kidnapped earlier this month by unidentified North Koreans have been freed, Chinese state media said, as diplomats worked to secure the release of the remainder.

The freed Chinese, whose number was not disclosed, were among a group of 29 fishermen working on three different boats who were reportedly captured at sea on May 8 by gunmen believed to be North Koreans.

"Some of the detained vessels and crew have already returned to port," the state-controlled Xinhua news agency said late Friday, without elaborating.

The foreign ministry was not immediately available for comment, and one of the owners of the boats that had been seized said he also had no information.

"I also saw the news online, but we haven't met any of the fishermen," the owner, Zhang Dechang, told AFP by telephone from the northeastern province of Liaoning, near China's border with North Korea.

"I called some of their relatives, and they didn't know either what the situation is. We still have to wait."

Chinese state media previously reported that the men were fishing off the country's northeast coast, in the waters between China and North Korea, when they were snatched.

The Xinhua report said Chinese embassy staff in the North Korean capital Pyongyang were negotiating the release of the rest of the fishermen.

The fishermen remaining in North Korean hands were "safe and sound" and had enough to eat and access to medical help, according to Xinhua.

Chinese fishermen regularly run into difficulties with the authorities of other countries as they fish in areas that are claimed by both China and its neighbours.

However, so far there has been no indication that the North Korean gunmen who seized the sailors were security forces from the isolated country -- which counts China as its sole major ally.