The United States will strengthen joint war capabilities with South Korea to defeat any possible invasion by North Korea with "overwhelming power", the new head of US forces in South Korea said on Tuesday. "As an alliance, we must be prepared to fight and win," General Walter L Sharp said in a ceremony as he took command of 28,500 US troops here from General Burwell B Bell. The change of command ceremony followed talks between US Secretary of Defence Robert Gates and South Korea's Defence Minister Lee Sang-Hee. "Our purpose is to continue to deter aggression on the Korean peninsula and, should deterrence fail, with immediate and overwhelming firepower [South Korea] and the US will defeat that threat," Sharp said. "We are ready to respond quickly and decisively against any attempts to threaten the security of [South Korea]," he said. Gates backed a troop promise made by President George W Bush at an April summit with South Korean President Lee Myung-Bak. Under a worldwide US troop realignment plan, the two countries agreed in 2004 to cut the number from 37,500 to 25,000 by the end of this year. But Bush and Lee Myung-Bak agreed at their summit to maintain American troops at the current level of 28,500 in South Korea, citing North Korea's continuing military threat. "We face a serious adversary across the DMZ [demilitarised zone] in the North," Gates said, adding that allied troops would be able to deal with any potential threat. Gates said he and his South Korean counterpart had discussed an expanded role for the alliance that could look beyond the Korean peninsula. "We exchanged views on a broad range of issues and future of the alliance and in particular looking beyond the peninsula. We are working together on regional issues," Gates said without elaborating. A senior US official said the defence chiefs talked about "steps towards greater participation of South Korea" globally such as in Iraq or Afghanistan – a taboo issue under the previous liberal governments in Seoul. "You wouldn't have heard that from the previous administration," the official told AFP, underlining the change in tone as a result of the new conservative government in Seoul. The United States has asked South Korea to send personnel to help rebuild war-battered Afghanistan but is not asking for troops, according to Seoul. South Korea sent 210 engineering and medical troops to Afghanistan in February 2002 and withdrew them last December. North Korea test-fired long-range missiles and staged a nuclear test in 2006. It has since reached an international disarmament deal to shut down its nuclear programme. Under a mutual defence treaty, the South Korean military comes under US command in times of war. But this is to change by 2012. Seoul and Washington have been allies since the 1950-53 Korean War.