Bush was speaking after the Security Council began an emergency session to discuss possible responses to the test.
Bush condemned North Korea's move as a "provocative act."
France's UN ambassador says the Security Council should pass a binding resolution on North Korea that includes the threat of sanctions.
Bush said the U.S. is still working to confirm North Korea's claims. The French Defense Ministry says France's nuclear agency has confirmed that North Korea set off a blast that measured about a half a kiloton, but it did not confirm that the blast was caused by a nuclear device.
Japan has sent planes over the Sea of Japan to monitor radiation levels.
(AP, AFP, Reuters TV)