The defense 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. US 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 with South Korean Defense Minister Lee Jong-sup, following their annual security talks in November in Washington, and was due to meet with President Yoon Suk-yeol before flying to the Philippines. The latest meeting comes as South Korea pushes to bolster confidence in American extended deterrence - its military capability, especially nuclear forces, to deter attacks on its allies. Nuclear-armed North Korea launched an unprecedented number of missiles last year, including intercontinental ballistic missiles (ICBMs) capable of reaching the US 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 for bringing back US tactical nuclear weapons or even a South Korean nuclear program, though Seoul officials dismissed such a possibility. In a joint statement, the defense chiefs said they had agreed to boost information sharing and joint planning. They also committed to expand the "level and scale" of this year's combined military exercises, and to deploy more US strategic assets, such as aircraft carriers and bombers. More than 28,500 American troops are stationed in South Korea as a legacy of the 1950-1953 Korean War, which ended in an armistice, rather than a peace treaty. Pyongyang has denounced the joint drills as proof of the allies' hostile intentions, and has staged its own military shows of force. Austin said his trip was aimed at deepening cooperation to tackle shared security challenges and reaffirm the US extended deterrence commitment to South Korea as "ironclad" at a time of heightened tension and provocation. "That's why the United States and the ROK are taking clear, meaningful steps to modernize and strengthen our alliance," Austin said in a special op-ed release on Tuesday by Yonhap news agency, referring to South Korea by the initials of its official name, the Republic of Korea. "So our adversaries and competitors know that if they challenge one of us, they are challenging the US-ROK alliance as a whole," he added. Lee has said the two countries will hold nuclear tabletop exercises in February under the scenario of North Korea's nuclear attacks, as part of efforts to improve joint nuclear planning and implementation and boost information sharing. Austin said the exercises are in line with the allies' talks to expand extended deterrence activities and mechanisms on the peninsula and in the region.