North Korea's launch of a long-range rocket caught the world by surprise after Pyongyang claimed technical issues had caused a delay.
North Korea declared Wednesday's launch of a rocket and satellite a success, The Associated Press reported.
South Korea and Japan confirmed the liftoff but were still assessing whether the launch succeeded.
A Barack Obama administration official, who was not authorized to speak publicly, said: "We noted the launch and we are monitoring the situation. We will have further official comment later." The official was not authorized to speak publicly and spoke on condition of anonymity.
On Saturday, North Korea had widened the dates during which it might conduct the launch of its Unha-3 rocket, citing a technical problem. Washington says the launch is a cover for testing technology for missiles that could be used to strike the United States.
"It was a surprise in terms of the timing," said Bruce Bennett, senior defense analyst with RAND Corp. think tank. "They had talked about postponing for a week. To recover so quickly from technical problems suggests they have gotten good at putting together a missile."
North Korea has also conducted two nuclear tests since 2006, deepening Western concern over its capabilities.
David Wright of the Union of Concerned Scientists said initial reports that debris from the rocket landed near the Philippines 14 minutes after launch suggested that the first two stages of the three-stage Unha-3 rocket may have worked.
The last launch of the Unha-3 in April failed about two minutes after takeoff but still drew UN Security Council condemnation and tightening of sanctions against North Korea.
The U.S., Japan and South Korea last week vowed to seek further Security Council action if the North conducted a launch. It remains to be seen if Russia and China, the North's main ally, will agree to further sanctions.
Bennett said North Korea also claimed successful launches of long-range rockets 1998 and 2006 that in fact failed, so their claims would have to be closely scrutinized.
But he added the fact they had proceeded with the launch Tuesday indicated they anticipated a better chance of success this time.
Victor Cha, a Korea expert at Georgetown University and former White House policy director for Asia, said a successful launch would be major national security concern for the United States.
He said there would still be technical hurdles for the North to overcome, particularly in terms of getting a rocket to re-enter the atmosphere, but it would mean that North Korea is able to launch a long-range ballistic missile -- the first rival state to the U.S. do so since the Soviet Union and China.