US President Donald Trump's defense secretary has warned North Korea of an "effective and overwhelming" response if Pyongyang chooses to use nuclear weapons.
It came as he reassured Seoul of steadfast US support at the end of a two-day visit.
"Any attack on the United States, or our allies, will be defeated, and any use of nuclear weapons would be met with a response that would be effective and overwhelming," Defense Secretary Jim Mattis said at South Korea's defense ministry.
Mattis' remarks come amid concern that North Korea could be readying to test a new ballistic missile, in what could be an early challenge for Trump's administration.
North Korea, which regularly threatens to destroy South Korea and its main ally, the United States, conducted more than 20 missile tests last year, as well as two nuclear tests, in defiance of UN resolutions and sanctions.
The North also appears to have also restarted operation of a reactor at its main Yongbyon nuclear facility that produces plutonium that can be used for its nuclear weapons program, according to US think tank 38 North.
"North Korea continues to launch missiles, develop its nuclear weapons program and engage in threatening rhetoric and behaviour," Mattis said.
North Korea's actions have prompted the United States and South Korea to respond by bolstering defenses, including the expected deployment of a US missile defense system, known as Terminal High Altitude Area Defense (THAAD), in South Korea later this year. The two sides reconfirmed that commitment on Friday.