The United States said Monday it takes North Korea's threat of a pre-emptive nuclear strike seriously but will not scale back planned military exercises that have outraged Pyongyang.

State Department spokesman John Kirby said the North's recent behavior -- conducting tests of a nuclear device and a long-range rocket -- had already led to new UN sanctions against Kim Jong-Un's regime.

"Kim Jong-Un has a choice he can make, which he clearly seems unwilling to make, which is to ratchet down the tension on the peninsula, to focus his resources and energy on the people of North Korea and on peace and security there in the region, rather than trying to up the ante with these kinds of comments," he said.

Earlier Monday, Pyongyang had warned of pre-emptive and "indiscriminate" nuclear strikes against South Korea and the United States as the two allies started the military exercises.

But the US military has downplayed the risk from the impoverished and isolated authoritarian regime's nuclear arsenal and is going ahead with a joint annual exercise with Seoul involving 300,000 South Korean and around 17,000 US troops.

Kirby said Washington's commitment to South Korea's security was "100 percent" and that North Korea's posturing only made the exercises, a regular event for decades, all the more important.

"There would not be as compelling a reason to improve alliance capabilities if Pyongyang wasn't so intent on raising the stakes on the peninsula and decreasing any sense of security or stability there," Kirby said.

"We do take those threats seriously and again call on Pyongyang to cease with the provocative rhetoric, cease with the threats and, quite frankly more critically, cease the provocative behavior."

Both Kirby and Pentagon spokesman Captain Jeff Davis said Washington and Seoul continue to move toward deployment of a sophisticated US missile defense system to South Korea -- another point of tension.

Davis said that the two countries signed "terms of reference" for the Terminal High Altitude Area Defense System (THAAD) deployment last week.