The UN Security Council will hold an emergency meeting on Wednesday in New York after North Korea claimed it had carried out a "successful" hydrogen bomb test, diplomats said.
The closed-door morning talks between the 15 member nations were called by the UN and Japan.
"While we cannot confirm at this time that a test was carried out, we condemn any violation of UNSC Resolutions and again call on North Korea to abide by its international obligations and commitments," US mission spokeswoman Hagar Chemali said.
She did not confirm the timing of the meeting, but the council usually begins official work at 10:00 am (local time).
If the test was indeed a hydrogen bomb, it would be more powerful than anything the North has ever tested, marking a significant increase in capabilities.
The hermit state previously tested far less powerful fission blasts generated by uranium or plutonium.
A hydrogen, or thermonuclear device, uses fusion in a chain reaction.
If North Korea's claim is confirmed, it would massively raise the stakes around its banned nuclear programme and likely trigger tougher international sanctions.
Pyongyang has carried out three previous nuclear tests - in 2006, 2009 and 2013 - which led to a series of sanctions from the UN.
Several UN resolutions ban the reclusive North from any nuclear activity or ballistic missile technology.
At the council, Pyongyang ally Beijing regularly tries to shield the North from condemnation or sanctions, while Washington repeatedly denounces the communist regime and its human rights violations.
Besides China, Russia could also be reluctant to back harsh sanctions against the North.
It has labelled previous US-backed resolutions passed by the UN General Assembly condemning human rights violations in North Korea as counterproductive.
Japan joined the council on January 1 under a two-year mandate as a non-permanent member.
Each of the North's three previous nuclear tests was followed by increased international sanctions targeting financial institutions and businesses linked to North Korean nuclear or ballistic activities.
