A remaining unknown is whether a test would be designed to show off a new weapon made from highly enriched uranium, the newest fuel the North is experimenting with, rather than the plutonium bombs that it tested, with mixed success, in 2006 and 2009.

In Japan, government officials said the three-stage rocket, which the North had said was carrying a communications satellite, appeared to fly for more than a minute after it was launched at 7:40 a.m. local time, then broke up at an altitude of 400,000 feet and tumbled into several pieces into international waters in the sea west of the Korean Peninsula. In Washington, the Pentagon said in a statement that the first stage of the rocket fell into the sea about 103 miles west of Seoul, and the remaining stages “were assessed to have failed and no debris fell on land.” It said the debris had never been a threat.

The launching has been politically problematic for the Obama administration, which only weeks ago completed an agreement with the North to provide food aid in return for Pyongyang’s agreement to suspend uranium enrichment and refrain from test launchings of long-range missiles. The administration had portrayed the deal as a promising if fragile advance that would allow nuclear monitors back into the country after years when the nuclear program continued unchecked.

Underscoring the political delicacy of North Korea in an election year, Mitt Romney, the presumptive Republican presidential nominee, said the launching illustrated President Obama’s strategy of appeasement. “This incompetence from the Obama administration has emboldened the North Korean regime and undermined the security of the United States and its allies,” he said in a statement.

The administration says it specifically told the North Korean negotiators that the deal was off if satellites were launched, since it considers such launchings a pretext for missile tests. But that requirement was not put in writing. Critics questioned the administration’s decision to go ahead without a written commitment, given the North Koreans’ history of breaking international agreements. But the administration insisted that it had not fallen into the same trap as past administrations — which made concessions only to have North Korea renege on deals — because the United States had not yet delivered the food aid.