By Jon Herskovitz

SEOUL, June 3 (Reuters) - North Korea presents a serious security threat and U.S. forces in South Korea are ready to respond quickly and decisively to counter any attack, the U.S. defence secretary and military leaders said on Tuesday.

The United States has been trying to convince impoverished North Korea to abide by an international disarmament deal and scrap its nuclear arms programme in exchange for aid and better global standing.

"We face a serious adversary across the DMZ (Demilitarised Zone) in the North. That is why we have this alliance," Defense Secretary Robert Gates told reporters at a ceremony to mark a change of command for U.S. military forces in South Korea.

The United States has about 28,000 troops in the country to support the South's 670,000-strong military. North Korea stations most of its 1.2-million-troop army near the DMZ buffer that has divided the peninsula since the end of the 1950-53 Korean War.

Gates and South Korean Defence Minister Lee Sang-hee met ahead of the ceremony and agreed to keep U.S. troop numbers at their current levels, the ministry said in a statement.

General Walter L. Sharp, who took over as commander of U.S. Forces Korea, said the allies were ready to deter North Korean aggression with "immediate and overwhelming firepower".

"We are ready to respond quickly and decisively against any attempts to threaten the security of the Republic of Korea (South Korea)," Sharp said.

Last Friday, North Korea, which has more than 1,000 missiles with at least 800 of them ballistic, rattled sabres by launching short-range missiles off its west coast.

Experts said North Korea, which tested a nuclear device in October 2006, developed an arsenal of short-range missiles to threaten the capital Seoul as well as South Korean and U.S. military bases near their heavily armed border.

North Korea, which wants to see U.S. soldiers removed from the peninsula, says it developed nuclear weapons to deter what it sees as a hostile policy from Washington.

"Never again can we allow the Republic of Korea to be unprepared for an attack," the outgoing U.S. commander, General B.B. Bell, said.

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