Washington ? The U.S. on Sunday claimed initial success two days into an assault on Libya that included some of the heaviest firepower in the American arsenal — long-range bombers designed for the Cold War — but American officials said Sunday it was too early to define the international military campaign’s endgame.

The top U.S. military officer suggested that Libyan strongman Moammar Gadhafi might stay in power in spite of the military assault aimed at protecting civilians, calling into question the larger objective of an end to Gadhafi’s erratic 42-year rule. Other top U.S. officials have suggested that a weakened and isolated Gadhafi could be ripe for a coup.

A second wave of attacks, mainly from American fighters and bombers, targeted Libyan ground forces and air defenses, following an opening barrage Saturday of sea-launched Tomahawk cruise missiles.

Defense Secretary Robert Gates said the U.S. expects to turn control of the mission over to a coalition — probably headed either by the French and British or by NATO — “in a matter of days.”

Late Sunday, however, NATO’s top decision-making body failed to agree on a plan to enforce the no-fly zone over Libya, although it did approve a military plan to implement a U.N. arms embargo.

At the Pentagon, Navy Vice Adm. William E. Gortney, staff director for the Joint Chiefs of Staff, told a news conference that the back-to-back assaults Saturday and Sunday had inflicted heavy damage. They largely silenced Gadhafi’s air defenses, blunted his army’s drive on the rebel stronghold of Benghazi and confused his forces.

“We judge these strikes to have been very effective in significantly degrading the regime’s air defense capability,” Gortney said. “We believe his forces are under significant stress and suffering from both isolation and a good deal of confusion.”

Gortney’s assessment suggested that further strikes on the scale of Saturday’s heavy assault with sea-launched Tomahawk cruise missiles may not be needed, although he did not rule out further attacks.

Gortney said Gadhafi himself is not a target, but he could not guarantee the strongman’s safety.

Inside Gadhafi’s huge Tripoli compound, an administration building was hit and badly damaged late Sunday. An Associated Press photographer at the scene said half of the round, three-story building was knocked down..

The systems targeted most closely were Libya’s SA-5 surface-to-air missiles, Russian-made weaponry that could pose a threat to allied aircraft many miles off the Libyan coastline. Libya has a range of other air defense weaponry, including portable surface-to-air missiles that are more difficult to eliminate by bombing.

Sunday’s attacks, carried out by a range of U.S. aircraft — including Air Force B-2 stealth bombers as well as Marine Harrier jets flying from an amphibious assault ship in the Mediterranean — demonstrated the predominance of U.S. firepower in the international coalition. By striking Libyan ground forces, coalition forces also showed that they are going beyond the most frequently discussed goal of establishing a no-fly zone over the country.

A military official said the B-2s flew 25 hours in a round trip from Whiteman Air Force Base in Missouri and dropped 45 2,000-pound bombs.

U.S. missiles and warplanes were clearly in the lead Saturday and Sunday, but Gates said the plan remains for the U.S. to step back once the threat from the Libyan military is reduced.

“We agreed to use our unique capabilities and the breadth of those capabilities at the front of this process, and then we expected in a matter of days to be able to turn over the primary responsibility to others,” Gates told reporters traveling with him to Russia. “We will continue to support the coalition, we will be a member of the coalition, we will have a military role in the coalition, but we will not have the preeminent role.”