South Korea and the United States launched major annual military exercises on Monday, as North Korea said it was ready for war over what it termed a reckless provocation.

"The reckless war drills targeting the army and people of the DPRK (North Korea), who are in the mourning period, amount to a grave provocation," said a foreign ministry spokesman, referring to the death of leader Kim Jong-Il.

The spokesman denounced the timing of the exercises, days after US and North Korean officials held talks in Beijing. The North "is fully ready for both dialogue and war", he told the official news agency.

The Key Resolve exercise which began Monday is a 12-day drill largely involving computer-simulated war games, while Foal Eagle from March 1 to April 30 involves field training for air, ground and naval forces.

The US military said in a statement the North had been informed a month ago of the exercise dates and "the non-provocative nature of this training."

But the official news agency, in a separate comment earlier Monday, described the drills as "an unpardonable infringement upon the sovereignty and dignity" of the North while it is still mourning Kim.

"The army and people of the DPRK are fully ready to fight a war with them," it said, warning Seoul and Washington of possible "catastrophic consequences".

The country's powerful National Defence Commission on Saturday denounced the drills as a "silent declaration of war".

The United States has based troops in the South since the 1950-53 war and the force currently numbers 28,500.

Pyongyang habitually fiercely denounces the annual joint exercises but has not physically responded to them. Seoul and Washington say they are merely defensive in nature.

The North has taken a hostile tone with the South since Kim died on December 17 and was replaced by his youngest son 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.

Jong-Un inspected two army battalions at a base near the border with the South, state media said Sunday, adding that one of them had staged the shelling of a South Korean island in November 2010 which killed four people.

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

The South's military has strengthened monitoring of the North's activities to guard against potential attacks, Yonhap news agency reported.

It said RF-4 and U-2 reconnaissance aircraft would be fully mobilised and F-15K fighter jets would be on emergency standby.

Artillery units near the land border would also stand ready to immediately hit back if attacked, it said.

The North's hostile comments came despite its talks in Beijing last week with the United States, about a possible resumption of six-nation nuclear disarmament negotiations.

Baek Seung-Joo of the South's Korea Institute for Defense Analyses said the recent hostile rhetoric is aimed at cementing solidarity among the North's elite while Pyongyang tries to mend ties with Washington.

"Jong-Un needs to prevent any divisions in domestic politics while the North-US negotiations are underway," he told AFP.

Baek said the North might launch a sudden retaliation after the exercise is over, but any such attack may be delayed until after the South's parliamentary election on April 11.

"The North wouldn't want a situation in which angry South Koreans refuse to vote for opposition parties seeking reconciliation with Pyongyang. So they will carefully time any attack to prevent such setbacks," he said.