The US, South Korea and Japan have said they will be united in their response to North Korea's claim to have successfully tested a hydrogen bomb.
North Korea said it carried out the test on Wednesday - if confirmed it would be its fourth nuclear test, and its first of the more powerful H-bomb.
The UN Security Council has also agreed to start drawing up new measures against North Korea.
But scepticism remains over whether the North really did conduct such a test.
Experts have said the seismic activity generated by the blast was not large enough for it to have been a full thermonuclear explosion.
What we know so far
The politics behind the claim
How to stage an underground test
Pyongyang's previous nuclear tests
North Korea's dramatic rhetoric
The White House said President Barack Obama had spoken separately to South Korea's President Park Geun-Hye and Japan's Prime Minister Shinzo Abe.
They "agreed to work together to forge a united and strong international response to North Korea's latest reckless behaviour", it said in a statement.
Mr Abe told reporters: "We agreed that the provocative act by North Korea is unacceptable. "We will deal with this situation in a firm manner through the cooperation with the United Nations Security Council."
South Korea's presidential office said in a statement that Ms Park and Mr Obama had agreed to closely co-operate and that the international community "must make sure that North Korea pays the corresponding price" for the nuclear test, reported Yonhap news agency.