South Korea's leader warned troops Friday to guard against North Korea's "reckless" military provocations, as a new dispute erupted between the two countries over four defectors from the communist state.

President Lee Myung-Bak stressed the need for separate branches of the military to work together to counter the threat from the North's special warfare forces, which Seoul says number 200,000.

"Through reckless military provocations, they (the North) are continuing to threaten peace," he told a multi-service officer commissioning ceremony at Gyeryongdae, 160 km (100 miles) south of Seoul.

The defector dispute is the latest episode in a year of high tensions and comes as US and South Korean troops stage major military exercises that the North has branded a rehearsal for invasion.

The South tried Friday to repatriate 27 North Koreans whose boat drifted across the border on February 5. But it says two men and two women who were on the boat chose to stay in the South -- a claim rejected by Pyongyang.

The North as of late afternoon had refused to send anyone to the frontier village of Panmunjom to accept the 27, apparently because it also wants the other four also to be returned.

A Seoul unification ministry spokesman said there was no word from the North on the transfer as of 6:00 pm (0900 GMT).

The communist state late Thursday accused the South of "despicable unethical acts" and said the group on the boat had been held hostage in a bid to fuel cross-border confrontation.

The North said their craft had drifted in fog and all those on board had demanded they be sent home. But Seoul had pressured them to remain in the South "by appeasement, deception and threat", it said.

"This cannot be interpreted otherwise than a grave provocation to the DPRK (North Korea)," said a statement attributed to the North's Red Cross.

Seoul's Unification Minister Hyun In-Taek told parliament the four had not been forced to stay. "We made a decision after respecting their free will," he said.

Won Sei-Hoon, the South's spy agency chief, told legislators separately that Seoul would not change its position "whatever North Korea may say".

The four include the 38-year-old boat captain. He apparently feared punishment if sent back and decided to stay when he saw how different life in the South is, Chosun Ilbo newspaper said.

Relations have been icy for months, after the South accused the North of torpedoing a warship in March 2010 near the disputed Yellow Sea border with the loss of 46 lives. It denies the charge.

Last November the North shelled a South Korean island near the border, killing two marines and two civilians.

The North is trying to shore up the position of Kim Jong-Un, youngest son of leader Kim Jong-Il, as eventual successor to his father. It also appears unnerved by pro-democracy protests sweeping the Arab world, analysts say.

Pyongyang has stepped up a clampdown on outside information to block news of the protests and prevent disturbances among its own people, Won told legislators.

A Seoul-based defector group says it will float leaflets and video footage with news of the Arab protests into North Korea next week, despite Pyongyang's threat to open fire on launch sites for the leaflets.

Activists protested in central Seoul against the threat, defacing and burning portraits of the Kims. "Shoot if you dare to. We will keep on launching leaflets," a banner read.