South Korea and the U.S. kicked off annual military exercises Monday, a day after North Korea denounced the training as a rehearsal for invasion and threatened to attack the allies.

About 18,000 American soldiers and an undisclosed number of South Korean troops are taking part in 11 days of drills across South Korea, according to U.S. and South Korean militaries.

The exercises, dubbed Key Resolve and Foal Eagle, are aimed at rehearsing how to deploy U.S. reinforcements in time of an emergency on the Korean peninsula, U.S. military spokesman Kim Yong-kyu said.

The U.S. and South Korea argue the drills — which include live-firing by U.S. Marines, aerial attack drills and urban warfare training — are purely defensive. North Korea claims they amount to attack preparations and has demanded they be canceled.

The North's military warned Sunday that it would bolster its nuclear capability and break off dialogue with the U.S. in response to the drills. It also said it would use unspecified "merciless physical force" to cope with them, saying it is no longer bound by the armistice that ended the 1950-53 Korean War.

South Korea's military has been closely monitoring Pyongyang's maneuvers but hasn't seen any signs of suspicious activities by North Korean troops, Seoul's Joint Chiefs of Staff said Monday.

Koh Yu-hwan, a professor at Seoul's Dongguk University, dismissed North Korea's statement as rhetoric. "The North's strong protest is not unusual as it also protested during previous drills," he said.

The training comes as the U.S. and other regional powers are pushing for the North to rejoin international disarmament talks on ending its atomic weapons program in return for aid. The North quit the six-nation weapons talks and conducted its second nuclear test last year, drawing tighter U.N. sanctions.

The North has demanded a lifting of the sanctions and peace talks with the U.S. on formally ending the Korean War before it returns to the negotiations. The U.S. and South Korea have responded the North must first return to the disarmament talks and make progress on denuclearization.

The U.S. stations about 28,500 troops in South Korea.