WASHINGTON (REUTERS) - US President Barack Obama condemned North Korea's nuclear test on Friday as a "grave threat" to regional security and international peace and said he would work with US partners on new sanctions against Pyongyang.

"To be clear, the United States does not, and never will, accept North Korea as a nuclear state," Obama said in a statement, in which he singled out Pyongyang as the only country to have tested nuclear weapons this century.

North Korea's launch on Friday of its fifth and biggest nuclear test also drew condemnation from its main ally, China. Pyongyang said it had mastered the ability to mount a warhead on a ballistic missile, ratcheting up a threat the international community has been powerless to contain.

Obama said the test was "a grave threat to regional security and to international peace and stability" and "follows an unprecedented campaign of ballistic missile launches."

The test was launched on the 68th anniversary of North Korea's founding, and came as Obama returned to Washington from a nine-day trip to Asia.

The US President said he had spoken by telephone with the leaders of South Korea and Japan, who agreed to work with other countries "to take additional significant steps, including new sanctions, to demonstrate to North Korea that there are consequences to its unlawful and dangerous actions."

Earlier in the week, North Korea fired three ballistic missiles into the sea off its east coast, the South Korean and US militaries said, as the leaders of the Group of 20 major economies held a summit in China.