North Korea on Thursday seized a stray South Korean fishing boat off the country's east coast, officials said, amid tensions over the communist state's nuclear programmes.

The South's government called for the swift return of the boat and its four-member crew, but there was no immediate response from the North.

The "800 Yeonan" was towed by a North Korean patrol boat after it strayed into the North's waters at 6:27am (2227 GMT Wednesday), the Joint Chiefs of Staff office said.

The 29-ton boat for catching squid was presumed to have sailed off its intended course at dawn because it had a problem with its global positioning system, the office said.

"We immediately radioed to the North Korean navy, requesting the return of the stray South Korean fishing boat, but there was no response," a Joint Chiefs of Staff office spokesman told AFP.

Seoul's unification ministry also demanded that the boat and its crew be returned "as soon as possible" in a telephone message to the North, spokesman Chun Hae-Sung told reporters at a briefing.

South Korean officials said the seized boat was being taken towards the North's eastern port of Jangjon.

The two countries, which still remain technically at war after the 1950-1953 Korean War, have a record of returning stray fishing vessels.

Two South Korean fishing boats — one in April 2005, and the other in December 2006 — returned home soon after accidentally straying into North Korean waters, according to Yonhap news agency.

Ties between the two Koreas warmed up after their first peace summit in 2000.

But cross-border ties have sharply worsened in the past year, and the North's prolonged standoff with the rest of the world over its nuclear and missile programmes has intensified in recent months.

Pyongyang has blamed the deterioration in relations on South Korea's conservative President Lee Myung-Bak, who took office in 2008 signalling a tougher stance towards North Korea.

The North has since March 30 held a South Korean worker at a Seoul-funded joint industrial site, accusing him of slandering its political system and trying to incite a local woman worker to defect.

The North quit six-nation nuclear disarmament talks after the UN Security Council censured it for a long-range rocket launch in April. In May it also staged its second nuclear test.

The Council has since imposed a series of tougher sanctions, including an expanded arms embargo and beefed up inspections of air, sea and land shipments going to and from North Korea.

The North has recently said it is open to a new form of dialogue with the United States on the nuclear issue outside the six-party format.

But Washington has called on the international community to continue to pressure North Korea to return to the six-party talks which also group South Korea, China, Japan and Russia.

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