The United States is willing to negotiate with North Korea, but only if it stops launching ballistic missiles, Secretary of State Rex Tillerson said Monday at the Association of Southeast Asian Nations regional conference in Manila.

"The best signal that North Korea could give us that they're prepared to talk would be to stop these missile launches," he said. "We've not had any extended period of time where they have not taken some type of provocative action by launching ballistic missiles. I think that would be the first and strongest signal they could send to us, would be to stop these missile launches."

The talks would be centered on denuclearizing Pyongyang without hurting its economy or military. But he's not putting a time frame on when those talks will or must occur, Tillerson said.

"We're not going to give someone a specific number of days or weeks," he added. "This is not 'Give me 30 days and we're ready to talk.' It's not quite that simple. It is all about how we see their attitude in approaching a dialogue with us."

Tillerson's comments came two days after the United Nations Security Council unanimously passed a resolution with severe sanctions against North Korea in response to its ballistic missile tests. Including a ban on coal, iron and lead exports, they are estimated to cost the isolated nation more than $1 billion in revenue – a loss of a third of the country's total exports.

U.S. Ambassador to the United Nations Nikki Haley called it "the single largest economic sanctions package ever leveled against the North Korean regime" and "the most stringent set of sanctions on any country in a generation."

President Donald Trump tweeted his approval for the resolution Sunday, saying he was "very happy and impressed with 15-0 United Nations vote on North Korea sanctions."

Less enthused, North Korea responded to the sanctions Monday through its state-run KCNA news agency with a threat to retaliate "thousands-fold" in an "action of justice" against the United States.