South Korean President Lee Myung-bak in January did publically call to repair relations with North Korea [EPA]

South Korea and the United States have begun a major military exercise, a Seoul spokesman said, despite North Korea's threats of a possible retaliation.

A spokesman for the south Korean Joint Chiefs of Staff said the annual Key Resolve drill computerised command post exercise had started and will continue until March 9.

In response, the North's National Defence Commission has denounced the annual drill -- which will be followed by joint air, ground and naval field training exercise from March 1 to April 30 -- as a "silent declaration of war".

"Our army and people will foil the moves of the group of traitors to the nation and warmongers at home and abroad for a new war with a sacred war of our own style," the commission, the North's top ruling body, said on Saturday.

The US military has 28,500 troops based in the South, saying the drills are just a defensive exercise. 

The North habitually denounces the annual exercises as a rehearsal for invasion.

The North has taken a hostile tone with the South since its leader Kim Jong-Il died in December 2011 and was replaced by his youngest son Kim Jong-Un.

The new leader has been appointed armed forces chief and has visited several units in an apparent attempt to burnish his military credentials.

Kim inspected two army battalions at a base near the border with the South, state media said on Sunday, adding that one of them had staged the deadly shelling of a South Korean island in November 2010.

The leader ordered "a powerful retaliatory strike" if Monday's drills intrude on North Korean territory, it said.

Pyongyang's state news agency on Monday described Key Resolve as "an unpardonable infringement upon the sovereignty and dignity" of the North while it is still in mourning for the late leader.

Cross-border tension has been high since Seoul accused Pyongyang of torpedoing a warship with the loss of 46 lives near the tense sea border off the west coast in March 2010.

The North denied involvement but went on to shell Yeonpyeong island later that year, killing four people and sparking brief fears of war.

Source: Agencies