North Korea has said it has placed its army on full combat alert in preparation for joint South Korea and U.S. military drills, which Pyongyang sees as nuclear war manoeuvers.

As the exercise started, North Korean state television criticized the drill saying it would boost its nuclear weapons capability because U.S. President Barack Obama was determined to ignore its calls for peace and to bring it down by military force.

More than 200 U.S. rangers and South Korean troops participated in a guerilla warfare exercise in Pohang, south of Seoul, with the units practicing their rappelling skills in the snow.

The drills, aimed at testing the allies´ defence readiness, draw fiery rhetoric from the North each year and fuel tensions on the Korean peninsula despite the fact they have been held for decades without major incident.

The North has come under pressure to return to six-country disarmament-for-aid nuclear talks because of U.N. sanctions imposed after a May 2009 nuclear test.

The sanctions have dealt a blow to its wobbly economy, and a botched currency move late last year has sparked inflation and rare civil unrest.

The two Koreas are technically still at war because their 1950-53 conflict ended with an armistice and not a peace treaty.

Key Resolve/Foal Eagle drills involve about 18,000 U.S. troops, with 8,000 coming from abroad and 10,000 being already stationed in the South.

The South´s Defence Ministry said about 20,000 of its troops will participate in the drills.

The United States, which fought on behalf of the South during the 1950-53 Korean war, has about 28,000 troops in the country to support its 670,000 soldiers.

The North´s deploys most of its 1.2 million troops near the border with the South.