BEIJING (AFP) -- Beijing said Tuesday it was investigating the seizure of 28 Chinese fishermen by armed North Koreans, as it gave its clearest indication yet that state forces may have played a role.

The men were seized as they fished in waters running between China and North Korea on May 8, and held for 13 days before they were released at the weekend, according to earlier reports.

Neither country has identified the abductors, and China has been unusually tight-lipped on the incident, which comes at a sensitive time in its relations with its nuclear-armed northern neighbour.

But Chinese media have pointed the finger at North Korea's coast guard and on Tuesday, the foreign ministry said Beijng had negotiated with Pyongyang to secure the fishermen's release.

"The Chinese side also urged the Democratic People's Republic of Korea to follow the relevant stipulations of the China-DPRK consular agreement to properly handle issues," spokesman Hong Lei told a regular news briefing.

That agreement includes the right to consular visits with its nationals, he said, but gave no further details.

"China's fishery department is carrying out an investigation into the details of this case," he added.

China is North Korea's key source of economic support, but Beijing has appeared at a loss as to how to rein in recent provocative behaviour by the isolated state.

The abductions came after Beijing criticised a recent North Korean rocket launch and expressed concern over a nuclear weapons test reportedly being readied by the isolated nation.

Chinese media originally said the men were taken by a group of armed "kidnappers" and put their number at 29, but this was later revised to 28.