(Adds remarks of S.Korean president; paragraphs 8,9)
By Hyonhee Shin
SEOUL, Jan 31 (Reuters) - The defence chiefs of the United States and South Korea vowed on Tuesday to expand military drills and boost nuclear deterrence planning to counter North Korea's weapons development and prevent a war.
U.S. Defense Secretary Lloyd Austin was in Seoul for talks as Washington seeks to reassure a key Asian ally over its nuclear commitment amid growing threats from North Korea.
Austin met South Korean Defence Minister Lee Jong-sup, following their annual security talks in Washington in November, and was set to meet President Yoon Suk-yeol before flying to the Philippines.
"In order to realise peace through strength on the Korean peninsula, we pledged to expand the scale and elevate the levels of the combined exercises and training," Lee told a joint news conference.
The latest meeting of the two came as South Korea pushes to bolster confidence in American extended deterrence - its military capability, especially nuclear forces, to deter attacks on its allies.
Austin said his trip aimed to deepen co-operation on tackling shared security challenges and reaffirm the U.S. commitment to South Korea as "ironclad" at a time of heightened tension and provocation.
"The United States stands firm in its extended deterrence commitment that includes the full range of U.S. defence capabilities, including our conventional, nuclear and missile defence capabilities," he told the conference.
Meeting with Austin later, South Korean President Yoon Suk-yeol stressed the need for discussions between the allies to evolve an effective and powerful system of extended deterrence, so helping to dispel the concerns of the South Korean people.
"North Korea's nuclear threats continue to escalate day by day," Yoon added, according to a statement from his office.
Nuclear-armed North Korea launched an unprecedented number of missiles last year, including intercontinental ballistic missiles (ICBMs) capable of reaching the U.S. mainland.
Officials from both the United States and South Korea have also warned Pyongyang could be preparing for its first test of a nuclear device since 2017.
The North's evolving threats have revived calls from some politicians and experts in South Korea to bring back U.S. tactical nuclear weapons or even a South Korean nuclear programme, though Seoul officials dismissed such a possibility.
In a joint statement, the defence chiefs added that they had also committed to expand this year's combined military exercises, and deploy more U.S. strategic assets, such as aircraft carriers and bombers.