China has asked North Korea to release 16 Chinese fishermen, who were onboard a Chinese fishing boat, which its owner says was seized by armed North Koreans on May 5, China’s state-run news agency Xinhua reported on Sunday.

The Chinese embassy in North Korea has asked Pyongyang to ensure the safety and interests of the fishermen, a Chinese counselor to North Korea said on Sunday, Xinhua reported.

Counselor Jiang Yaxian said a private fishing boat from Dalian in Northeast China's Liaoning Province, known as Liaoning Generic Fishing No. 25222, was seized by the North Koreans.

Jiang said Yu Xuejun, the boat’s owner, called the embassy for help on May 10.

“Upon receiving the call, the Chinese embassy promptly made representations to the ... DPRK [North Korea] Foreign Ministry, asking the DPRK side to release the boat and the fishermen as soon as possible,” Xinhua quoted counselor Jiang as saying.

The seizure is likely to affect the strained ties between Pyongyang and its longstanding ally China. Some Chinese banks have frozen transactions with North Korea's main foreign exchange bank, as frustration builds in China over Pyongyang's defiance in going ahead with its nuclear weapons programs. Pyongyang’s nuclear test in February was unanimously condemned by the U.N. Security Council with a statement that was approved by all 15 member nations, including China.

The boat's owner Yu told the Global Times that he had received eight calls from the captors, who demanded 600,000 yuan ($97,000) as ransom. He said the vessel had been in Chinese waters when it was captured.

“The ship was equipped with GPS and Beidou positioning systems (a Chinese satellite navigation system) to make sure our fishermen know their accurate location every second,” Yu said, adding that the Liaoning fishery authority monitors all fishing activities and sends a warning as soon as any boat nears the North Korean waters.

Yu said the kidnappers removed the positioning systems and confiscated all communication devices after boarding the boat.

He said the food supplies on the boat would last only for 20 days and was likely to run out soon.

Last May, 29 fishermen from Liaoning were abducted by North Koreans while trawling and were released two weeks later, after the Chinese government’s intervention. When they returned home, Chinese media reported that the fishermen held in captivity were beaten and starved while in detention.