China says it has called on North Korea to secure the release of a fishing boat and its crew seized earlier this month.

Owner Yu Xuejun said the vessel had been in Chinese waters when the 16-man crew were seized on 5 May.

Mr Yu said the North Korean captors were demanding a 600,000 yuan ($100,000, £66,000) ransom.

State-run Xinhua news agency said that diplomats in Pyongyang had been asked for help on 10 May and were working on the issue.

"Upon receiving the call, the Chinese embassy promptly made representations to the... DPRK [North Korean] Foreign Ministry, asking the DPRK side to release the boat and the fishermen as soon as possible," the agency quoted Counsellor Jiang Yaxian of Beijing's embassy in Pyongyang as saying.

It called for the crews' "legitimate rights and interests" to be safeguarded, he added.

Boat-owner Mr Yu told Global Times newspaper he had received eight calls from the people holding his crew demanding the ransom.

There have been incidents in the past in the Yellow Sea, which lies between China and the Korean peninsula and has rich fishing grounds.

In May 2012, 29 Chinese fishermen and three vessels were seized by unidentified North Koreans.

They were freed after two weeks and it was not clear whether a ransom had been paid, nor whether the captors had been the North Korean authorities or autonomous kidnappers.

'Resume dialogue'

China has traditionally been North Korea's closest ally. But in the wake of Pyongyang's third nuclear test, on 12 February, ties between the two have chilled.

Beijing backed expanded sanctions on Pyongyang over the test and some of its banks have suspended transactions with North Korea's main foreign exchange bank.

Chinese state press has also become more vocal on the issue, openly debating the merits of alliance with Pyongyang.

Since Saturday, North Korea has fired five short-range missiles off its coast: three on Saturday, and one on both Sunday and Monday.

"North Korea again launched what appears to be a KN-02 short-range missile," a defence ministry official told South Korea's Yonhap news agency on Monday.

"We are closely watching the movements of the North's military in case of further launches."

The tests also prompted a warning from UN Secretary-General Ban Ki-moon on Sunday.

"I hope that North Korea will refrain from such actions," Mr Ban, who was visiting Moscow, told Russia's RIA Novosti news agency. "It is time for them to resume dialogue and lower the tensions."

North Korea routinely test-fires these kinds of missiles, but such a sustained launch period is unusual, the BBC's Lucy Williamson reports from Seoul.

There is worry in South Korea that, after suspending the joint Korean economic zone Kaesong Industrial Complex, and restarting its mothballed nuclear facility, North Korea is planning further actions that may ignite an international response, our correspondent adds.