Unidentified North Koreans have detained 29 Chinese fishermen from three boats and are demanding payment for their release, Chinese media says.

They were seized on 8 May in the Yellow Sea between China and North Korea by a gunboat, said the Beijing News.

The captors have asked for payment by Thursday for the release of the men and boats, the newspaper reported.

China's foreign ministry said it was in touch with North Korean authorities and hoped to resolve the situation soon.

"We urged the North Korean side to guarantee the legal rights of the Chinese fishermen," the ministry's spokesman Hong Lei said.

He refused to confirm if the payment being demanded was a ransom, said an Associated Press report.

It is not clear if the boats were seized by North Korean authorities or kidnappers as some reports have suggested.

Pyongyang has not commented on the incident.

The owners of the boats have been reported as saying that the men were fishing in Chinese waters when the incident took place.

One of the owners, Zhang Dechang, told the Beijing News that the captors initially demanded payment of 1.2 million yuan (£119,300, $189,800) for the return of the men and boats, now reportedly in North Korea.

The captors then reportedly reduced their demand to 900,000 yuan.

Mr Zhang said that the captors included both North Koreans and Chinese, according to a Global Times report.

''They had guns; no one resisted. The captured fishermen have been locked in a small house, with no food to eat,'' he was quoted as saying in the report.

China is North Korea's closest ally. But the waters of the Yellow Sea, home to rich marine life, have seen clashes in the past between vessels from China and the two Koreas.