TRIPOLI, Libya — Four days of allied strikes have battered Libyan leader Moammar Gadhafi’s air force and largely destroyed his long-range air defense systems, a top U.S. commander said Tuesday. But there was little evidence that the attacks had stopped regime forces from killing civilians or shifted the balance of power in favor of the rebels.

Gadhafi loyalists made further advances into the besieged western city of Misurata, continued to pound the small town of Zintan southwest of Tripoli, the capital, and fired artillery to hold at bay the rebels attempting to regroup outside the strategic eastern town of Ajdabiya.

The Libyan military’s attacks and the mounting civilian deaths call into question whether the internationally imposed no-fly zone can achieve its goal of protecting civilians, let alone help loosen Gadhafi’s grip on power. It seemed unlikely that the coalition, which has argued in recent days over the scope and leadership of the allied mission, would countenance a significant escalation.

A U.S. fighter jet on a strike mission against a government missile site crashed Monday night in eastern Libya, about 25 miles outside the rebel capital of Benghazi. Both crewmen ejected safely and were rescued after the aircraft spun from the sky during the third night of the U.S. and European air campaign.

The crash, which the U.S. attributed to mechanical failure, was the first major loss for the U.S. and European military air campaign.

Late Tuesday, Gadhafi made his first televised appearance since the bombing campaign began, delivering a defiant address to supporters at his Tripoli compound, which was struck by Tomahawk missiles a few days earlier. “I am here, I am here, I am here,” he said, as celebratory gunfire echoed across the city. “We will win. We will be victorious in this historic battle.”

Heavy anti-aircraft fire and loud explosions sounded in Tripoli after nightfall, possibly a new attack in the international air campaign that so far has focused on military targets.

One of Gadhafi’s sons may have been killed, U.S. Secretary of State Hillary Clinton told ABC News on Tuesday. She cited unconfirmed reports and did not say which son she meant. She said the “evidence is not sufficient” to confirm this.

President Obama, meanwhile, sought to shore up support for the international mission, saying that the U.S. and allied efforts to halt advances by Gadhafi’s forces had “saved lives.”

“In Benghazi, a city of 700,000 people, you had the prospect of Gadhafi’s forces carrying out his orders to show no mercy,” Obama said at a news conference while in San Salvador. “That could have resulted in catastrophe in that town.”

Obama defended U.S. involvement against criticism from several members of Congress, including some fellow Democrats. Members of Maine’s congressional delegation said they have concerns about the U.S. involvement in Libya.

But Obama said that “it is in America’s national interests to participate … because no one has a bigger stake in making sure that there are basic rules of the road that are observed, that there is some semblance of order and justice, particularly in a volatile region that’s going through great changes.”

Hours earlier, a top U.S. military official had touted the limited gains that allied forces had made over the course of the four-day-old military intervention.

Since the bombing began Saturday, U.S. and allied forces have launched 162 Tomahawk missiles and conducted more than 100 attacks with precision-guided satellite bombs, said U.S. Navy Adm. Samuel Locklear, commander of the allied task force charged with enforcing the U.N. resolution that authorized action in Libya.

But he conceded that the airstrikes have been unable to halt attacks by Libyan government forces against civilians.

A doctor at a Misurata hospital said about 80 people had been killed in the city since the adoption Thursday of the U.N. resolution, which called for a halt to attacks on civilians. Among the 12 said to have died Tuesday was a family of six, killed when a tank shell hit their car. The doctor said he had stopped counting the injured, that patients are being treated on the floor and that the hospital is running out of almost all medicines and supplies.

“This no-fly zone doesn’t mean anything to us because Gadhafi only had a few planes and they were doing nothing,” said the doctor, who spoke by telephone on the condition of anonymity because he fears Libyan forces may soon retake the city. “We need a no-drive zone because it is tanks and snipers that are killing us.”

In Moscow, Defense Secretary Robert Gates said Tuesday that the pace of attacks would wane in the days ahead as the United States hands over responsibility for maintaining the no-fly zone to its allies and the number of clear targets diminishes.

Meanwhile, there were indications that international support for the coalition effort is beginning to flag, with China joining Russia in calling for a cease-fire to avert feared civilian casualties. China, like Russia, abstained from voting on the U.N. resolution.

“The U.N. resolution on the no-fly zone over Libya aimed to protect civilians,” Chinese Foreign Ministry spokeswoman Jiang Yu told reporters at a news briefing. “We oppose abuse of force causing more civilian casualties.”

U.S. and other coalition officials dispute Libyan assertions that the strikes have caused civilian deaths. “It’s perfectly evident that the vast majority — if not nearly all — of civilian casualties have been inflicted by Gadhafi,” Gates told reporters after meeting with Russian Defense Minister Anatoly Serdyukov. “We’ve been very careful about this.”

Clinton suggested Tuesday that the Libyan leader and some members of his inner circle might be searching for a way out of the country — and the conflict.

“We’ve heard about other people close to him reaching out to people that they know around the world — Africa, the Middle East, Europe, North America, beyond — saying what do we do? How do we get out of this? What happens next?” Clinton said in an interview with ABC News.

But at least in Tripoli, the government appears to be in firm control nearly a month after the last major protests were crushed by security forces using live ammunition.

The Associated Press contributed to this report