SEOUL (Reuters) - A nuclear-capable U.S. B-52 bomber sortie over South Korea has endangered plans for reunions between families from the North and South of the country and risks triggering a further escalation of military tension, North Korea said on Thursday.

North Korea said a flight by the nuclear-capable B-52 took place off the west coast of the Korean peninsula on Wednesday.

A U.S. military spokeswoman said she could not discuss details of specific missions, adding: "The U.S. Pacific Command has maintained a rotational strategic bomber presence in the region for more than a decade."

A South Korean military source told the Yonhap news agency that the flight was a training sortie involving a single aircraft. The North's National Defence Commission, the country's top military body, said in a statement read on state television, that it was a rehearsal for a nuclear attack.

"At the time when the agreement was made on reunions of separated families and relatives at Panmunjom, a formation of U.S. B-52 strategic bombers from Guam was carrying out nuclear strike practices all day over Korea's west sea, aiming at us," a spokesman for the Commission was quoted as saying.

In a rare confidence-building move, the two Koreas agreed on Wednesday, in talks at the border village of Panmunjom, to allow families still divided by the 1950-53 Korean War to meet for five days in late February for the first time since 2010.

South Korea's Ministry of Unification said it would be "regrettable" and would hurt separated families if North Korea did not go ahead with the reunions as agreed in response to the flight.

A sharp escalation of tension between the North and South in early 2013 triggered threats by the North of a nuclear strike on South Korea, Japan, the U.S. South Pacific territory of Guam and even the continental United States. Washington responded with B-2 and B-52 flights over South Korea.

Both aircraft can carry nuclear weapons.

The North has made a diplomatic push to try and halt U.S. and South Korean drills that are regularly staged at this time of year, although the South said on Thursday the drills would go ahead as planned.

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Hazel Smith, a North Korea expert at Britain's University of Central Lancashire said she expected North Korea to respond to the sortie.

"Since North Korea is motivated by military thinking, a show of force is probably going to provoke a response," she said.

Smith said any one of various groups in North Korea might try to make a show of power with a reaction to the flight, and young leader Kim Jong Un was probably unable to influence events.

"There is no political manager at the top of this - you need someone in an authoritarian country to manage the elite, and Kim Jong Un does not have the legitimacy, authority or experience to manage these different interests," Smith said.

"I have never seen such an internally unstable political situation in North Korea," said Smith, who has been researching the country for more than 25 years.

(Reporting by Ju-min Park and James Pearson; Editing by David Chance and Robert Birsel)