South Korea and the United States on Monday morning kicked off their annual joint military drills amid continuing tensions on the peninsula. The 11-day joint maneuver, codenamed Key Resolve/Foal Eagle, draws some 200,000 South Korean and 12,800 U.S. troops.

South Korean officials said a U.S. aircraft carrier would join one of the two exercises, while U.S. military officials here neither confirmed nor denied it, Yonhap News Agency reported.

Key Resolve, mainly involving computer simulations, will last until March 10, while Foal Eagle, involving joint air, ground and naval training exercises, will run through April 30.

The exercises focus on raising the allies' ability to defend against small, sudden attacks by the Democratic People's Republic of Korea (DPRK), the Combined Forces Command said, adding the drills are defensive in nature, aimed at strengthening the allies' readiness against all potential threats.

The drills come a day after the DPRK threatened all-out war in response to the joint drills by South Korea and the U.S., and told Seoul to stop sending anti-DPRK leaflets across the border.

Pyongyang would respond to the joint drill, with "unprecedented all-out counteraction" that would turn Seoul into a "sea of flames, " the official Korean Central News Agency said Sunday.

It also vowed to fire into South Korea if the South continues to drop balloons into the DPRK that are loaded with leaflets about democracy protests sweeping across the Middle East.