A top envoy from the U.S. State Department announced Tuesday that Libya’s rebel government would open an office in Washington, the latest indication that the United States views the rebels as the legitimate representative of the Libyan people.

The move coincided with an intensified NATO air campaign against the government of Moammar Gaddafi. Late Tuesday in Tripoli, at least seven explosions were heard, and dark smoke was seen billowing from the vicinity of Gaddafi’s compound in the capital. The sounds of gunfire also erupted from the area around the compound, but there was no immediate official explanation.

In the early morning hours, Tripoli shook with at least 15 massive explosions as NATO launched its largest airstrikes to date against the Gaddafi regime. The overnight bombings also appeared concentrated on the Libyan leader’s compound. NATO said in a statement that it had struck a vehicle storage facility adjacent to the compound.

“This facility is known to have been active . . . resupplying the regime forces that have been conducting attacks against innocent civilians,” NATO said.

Government spokesman Moussa Ibrahim said NATO had targeted the headquarters of the military reserves, killing at least three people and injuring dozens. He said the casualties would have been higher except that the government had emptied the headquarters, expecting that it would be hit. Still, he said, “we’ve never had such an injured number of people.”

Gaddafi was still alive, Ibrahim said.

Flashes lighted up the Tripoli sky near the compound, followed by explosions and shock waves that shook windows at a hotel a mile away. Antiaircraft gunfire punctuated the quiet between the strikes, and an acrid smell filled the air.

At Tripoli Central Hospital, where government minders took journalists shortly after the blasts, about a dozen injured people moved through a ward, many appearing to have light or moderate injuries to their limbs. All were men wearing civilian clothing. Most appeared to be in their 20s or 30s.

In another room were three dead men covered in dust. Each man’s head was partially blown away. Their bodies, though cut and bruised, were not damaged nearly as much.

In Benghazi, U.S. Assistant Secretary of State for Near Eastern Affairs Jeffrey Feltman stopped short of formally recognizing the rebel Transitional National Council as Libya’s governing body. Rebel officials have sought such recognition for weeks now, and other countries — notably France — have granted it.

But Feltman said such recognition was beside the point and emphasized that the United States was firmly committed to working with the rebels. He said he delivered to the rebel leaders an oral message from President Obama as well as a formal invitation to open the office, which Feltman said the rebel leaders had accepted.

“The point is Gaddafi and Tripoli are increasingly isolated diplomatically,” Feltman said. “In attacking, threatening and brutally suppressing the Libyan people, Colonel Gaddafi has lost the legitimacy to rule. He can’t regain control of Libya. He must step down immediately, thereby allowing the Libyan people to determine their own future.”

Libya’s conflict is currently bogged down in a military stalemate, with this oil-rich North African nation essentially split in half. The rebels, poorly trained and ill-equipped, are hoping that NATO bombardments will weaken Gaddafi’s forces and allow them to push westward toward Tripoli. Rebel leaders are also seeking financial support from the United States and European powers, including access to billions of dollars of Gaddafi’s frozen assets.

Feltman, who met with senior rebel leaders and civil society groups in the rebel stronghold of Benghazi, is the highest-level U.S. official to visit eastern Libya since the rebellion was launched in February.

“We’re not talking to Gaddafi or his people. They are not talking to us,” Feltman said. “We have American diplomats posted in Benghazi dealing with the council. We have no American diplomats in Tripoli.”

On Monday, European Union foreign policy chief Catherine Ashton visited Benghazi and pledged support for the rebel leadership.

Feltman said the United States had so far given $53.5 million to address Libya’s humanitarian needs and another $25 million in “non-lethal military supply.” He assured rebel leaders that more assistance will be coming in the weeks ahead.

“President Obama has proposed to Congress ways by which we can unfreeze Libyan assets in order to provide assistance and help to the Libyan people,” said Feltman, adding that such assets would be used to address the humanitarian needs of the Libyan people. “We’re in consultation with the U.S. Congress now on how we can move forward.”

NATO at the end of March took over a U.N.-mandated mission to protect civilians from attacks by Gaddafi loyalists, continuing a campaign of airstrikes and missile attacks begun by a U.S.-led Western coalition 12 days earlier. Allied military officials have spoken in recent weeks of the need for escalation and have called for Gaddafi to step down. The officials say they fear a stalemate is emerging, with Gaddafi remaining in power in the west, rebels controlling the east and a contested area in between.

Libyan officials accuse NATO of picking sides in a civil war and complain that strikes on Gaddafi’s Tripoli compound are attempts to assassinate the leader of a sovereign country.

On Monday, French officials said France and Britain planned to deploy attack helicopters to Libya as part of the NATO mission. Such a move would allow greater accuracy in military action within cities but would probably put their troops at higher risk.

The French defense minister, Gerard Longuet, told reporters in Brussels that the helicopters would be used against Libyan military equipment while trying to avoid civilian casualties, the Associated Press reported.

But British Armed Forces Minister Nick Harvey told parliament on Tuesday that the British government had made no final decision about deploying helicopters, Reuters reported. “It is an option which we are considering and at some point in the future we may get to the point of deciding to go down this route,” he said.

Longuet told reporters that France would use Gazelle helicopters, the Associated Press reported. He said British military officials were on “exactly the same wavelength” as the French.

During fighting in Misurata, a rebel-held city in western Libya that is under siege by government forces, the Libyan military moved into crowded areas, making it difficult for NATO to strike targets without risking significant civilian casualties. Helicopters would make it easier to attack military forces in those situations.

Improved accuracy is the goal of the helicopter deployment, French Foreign Minister Alain Juppe told reporters in Brussels.

No NATO military personnel have died in Libya during the operation, and there have been relatively few civilian deaths, something that even Libyan officials privately acknowledge.

Birnbaum reported from Tripoli. Staff writer Joby Warrick in Washington and special correspondent Karla Adam in London contributed to this report.