US President Barack Obama spoke by phone late on Monday with his South Korean counterpart Lee Myung-bak and Japanese Prime Minister Taro Aso to "coordinate" reaction to North Korea's nuclear test, the White House said.
Obama spoke with Lee "to consult and coordinate our reaction to the North Korean nuclear test," the White House said.
They "agreed to work closely together to seek and support a strong United Nations Security Council resolution with concrete measures to curtail North Korea's nuclear and missile activities."
In his talks with Aso, Obama reiterated his country's "unequivocal commitment to the defense of Japan and to maintaining peace and security in Northeast Asia."
Chief Japanese government spokesman Takeo Kawamura said the pair "agreed to make the international community's intention clear through adoption of a strong resolution at the Security Council."
Kawamura did not say whether the leaders would ask for new sanctions.
The UN Security Council unanimously condemned North Korea on Monday hours after it tested a nuclear bomb, with major powers vowing new punitive action against Pyongyang for violating the world body's resolutions.
Several western diplomats emerged from the emergency UN Security Council session hinting they would seek fresh sanctions against Pyongyang under a new resolution.
The US ambassador to the United Nations, Susan Rice, said "the United States will seek a strong resolution with strong measures," calling the test "a grave violation of international law, and a threat to regional and international peace and security."
But Rice stopped short of mentioning possible sanctions.
North Korea's test - an underground blast far bigger than its first nuclear test in 2006 - drew stern rebukes from global leaders, with Obama leading the pack with a warning of "grave" danger from the development.