South Korea Monday kept its frontline troops on alert but vowed to respond calmly after North Korea's military threatened all-out confrontation with Seoul.

The Joint Chiefs of Staff office said border army, navy and air force units remained on guard after an alert was ordered Saturday evening, but no unusual movements been detected in the communist North.

Frontline radar and reconnaissance planes have been closely monitoring military activities around the clock near the border, it said.

"The government is calmly responding, preparing for various possibilities but not making rash forecasts about future developments," unification ministry spokesman Kim Ho-Nyoun told a briefing.

On Saturday a spokesman for North Korea's army General Staff called for an "all-out confrontational posture" against South Korea, citing its alleged violations of a disputed border in the Yellow Sea.

Kim said the statement, announced by a uniformed officer on state TV, was the first from the General Staff since 1998.

The North's spokesman warned it would not allow South Korean intrusions into the area, the scene of bloody naval clashes in 1999 and 2002.

The sea border was unilaterally drawn by United Nations forces after the 1950-1953 war but the North refuses to accept it.

The Seoul government has made no official response to the military statement but political parties and newspapers in the South have denounced the threats.

Also Saturday Pyongyang's foreign ministry spelt out a hardline stance in nuclear disarmament negotiations, saying it may not give up its atomic weaponry even if it establishes diplomatic ties with Washington.

The ministry said it would retain its weapons as long as it feels under nuclear threat from the United States.

Some analysts say the statements aim to catch the attention of the incoming US administration rather than Seoul. They say Pyongyang wants to persuade Washington not to sideline it despite a daunting array of other global problems.

The North has been involved in the six-party talks since 2003. In 2007 it struck a deal that envisages denuclearisation of the Korean peninsula, diplomatic ties with Washington and a formal peace treaty.

But the last round of negotiations -- involving the two Koreas, China, the United States, Russia and Japan -- ended fruitlessly in December.

A South Korean team has been visiting North Korea since last week to discuss the possible purchase of unused fuel rods as part of measures to shut down plutonium-producing plants at Yongbyon.

Officials said the six-member delegation visited Yongbyon on Friday and would stay in Pyongyang until Monday for talks.