The Obama administration warned of "costly consequences" for North Korea on Thursday, in the wake of Pyongyang's threat to launch a pre-emptive nuclear strike on the US and its recent nuclear test.
Speaking at a Senate hearing, the US State Department's special representative on North Korea said Washington would not engage in negotiations without a "fundamental change in attitude" from the pariah state.
The Senate foreign affairs committee chairman, Robert Menendez, described the threat of a nuclear strike as "absurd and suicidal".
The hearing was taking place as the United Nations security council voted in New York to expand sanctions against North Korea. The US-drafted resolution was approved unanimously by the 15-nation council. It came after three weeks of negotiations between the US and China after North Korea's latest nuclear test on 12 February.