Seoul, South Korea (CNN) -- The United States and South Korea began their annual joint military exercise Monday, kicking off an 11-day drill designed to ensure the countries' alliance is "fully prepared to respond to any potential provocations," according to a statement from U.S. forces in South Korea.
On Monday, South Korean president Lee Myung-bak ordered his military to "thoroughly" prepare with the United States to counter threats from North Korea, the Yonhap news agency reported.
The timing of the joint training comes during a time of increased military tensions. South Korea blamed North Korea for sinking a 1,200-ton warship in March, killing 46 sailors. North Korea denied it sunk the ship.
North Korea said Sunday that it will "deal a merciless counterblow to the U.S. imperialists" and to South Korean "traitors," in response to the U.S.-South Korea joint military exercise.
"The military counteraction of the DPRK [Democratic People's Republic of Korea] will be the severest punishment no one has ever met in the world," a spokesman for the General Staff of the Korean People's Army said in a statement carried by North Korea's official news agency.
North Korea's statement Sunday took direct aim at U.S. President Barack Obama.
"The Obama Administration would be well advised to cool its head heated with the moves to isolate and stifle the DPRK," the spokesman for the General Staff of the Korean People's Army said, "and deeply ponder over what the DPRK meant when it clarified its resolute determination and solemn declaration to defend the country and the nation."
In response, Pentagon Spokesman Bryan Whitman said, "These are annual exercises that reflect the U.S. commitment to the stability and security of the peninsula."
On Sunday, Yonhap reported the South Korean president proposed a plan to reunite his country with North Korea and suggested a tax to prepare for the costs expected if the two countries come together.
"Inter-Korean relations demand a new paradigm," Lee said in a speech to mark the 65th anniversary of Korea's independence from Japanese colonial rule, Yonhap reported.
Lee said the two countries should form a "peace community," and stressed the importance of the denuclearization of North Korea, Yonhap reported. He added that inter-Korean exchanges are needed to help build North Korea's economy, according to Yonhap.
"It is imperative that the two sides choose coexistence instead of confrontation, progress instead of stagnation. The two of us need to overcome the current state of division and proceed with the goal of peaceful reunification," Lee said, according to Yonhap.