U.S. warns North Korea of 'overwhelming' response if nuclear arms used

"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 U.N. resolutions and sanctions.

The North 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 U.S. think tank 38 North.

"North Korea continues to launch missiles, develop its nuclear weapons program and engage in threatening rhetoric and behavior," 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 U.S. missile defense system, known as Terminal High Altitude Area Defense (THAAD), in South Korea later this year.

Once fully developed, a North Korean ICBM could threaten the continental United States, which is about 9,000 km (5,500 miles) from North Korea. ICBMs have a minimum range of about 5,500 km (3,400 miles), but some are designed to travel 10,000 km (6,200 miles) or more.