The military chief of staff for the Libyan opposition was killed Thursday by assailants in the rebel capital, creating chaos among the fractious coalition trying to overthrow Moammar Gaddafi.

The titular head of the rebels’ Transitional National Council announced at a news conference that Gen. Abdul Fattah Younis and two other senior opposition commanders had been fatally shot in Benghazi. It was not clear who was responsible for the deaths.

Younis, a former Gaddafi security minister and confidant, defected from the Tripoli regime with great fanfare and joined the rebel side in February, becoming part of the military leadership guiding the fight against Gaddafi’s forces. The general was popular among NATO officials and Middle East governments that support the Libyan revolution.

Although details remained murky, Younis’s death could shake international support for the opposition and rattle the divided rebel coalition, which this month gained U.S. recognition as Libya’s sole governing authority, while also strengthening Gaddafi’s resolve.

Younis was cheered when he defected, but he had many enemies. The general was distrusted by many rebels and dogged by accusations that he was still working for Gaddafi. But the Tripoli regime also hated him for what it viewed as his betrayal.

Libyan rebel military leader Abdel-Fattah Younis is greeted by Libyan rebels in April. Younis, the head of the Libyan rebel's armed forces, and two of his aides were killed by gunmen on Thursday. (Altaf Qadri/Associated Press)

In Washington, a senior administration official said the White House was aware of the reports of Younis’s death and was working to gather details.

Rebel officials provided few details about his death. Adding to the intrigue surrounding the case, Mustafa Abdel Jalil, the Transitional National Council leader, said rebel forces had been unable to locate the three bodies.

But a security officer told the Associated Press before Abdel Jalil’s announcement that officials had found three badly burned bodies and that one was known to be Younis’s.

Abdel Jalil did not name the assassins, but he said arrests had been made. Dozens of semi-independent militias and other armed groups operate in rebel-controlled eastern Libya.

Abdel Jalil said Younis had been summoned to Benghazi to answer questions about “a military matter” before he was killed, raising the possibility that rebel forces might have been involved in his death.

Reuters reported that rebel security forces had arrested Younis and two of his aides early Thursday from their operations center near the opposition’s eastern front. Security officials said at the time that Younis was to be questioned about allegations that his family still had ties to Gaddafi’s government.

Abdel Jalil called Younis a hero of the revolution. The general was a charismatic centerpiece of the rebel government and, being a defector from Gaddafi’s inner circle, he was important as a symbol. As a top military commander, he led thousands of rebel troops, though his leadership was controversial as the advance bogged down in the east.

News of Younis’s killing came as rebel forces descended from the western mountains onto the plains south of Tripoli on Thursday, capturing a string of strategic towns from troops loyal to Gaddafi.

Rebels take three towns

The rebel victories ended weeks of stalemate in the six-month-old conflict and gave opposition militias complete control of a vital highway that carries food, fuel and more fighters into the western mountains via Tunisia.

The running gun and rocket battles from town to town occurred south of the gateway city of Nalut along Libya’s border with Tunisia and followed strategic NATO airstrikes against Gaddafi’s artillery units.

“It was a decisive battle. We defeated them,” said Abdelaziz Mohammad Sayid, who stood beside the bodies of two Gaddafi soldiers partly covered by blankets in the afternoon sun in the town of Hawamid.

The advance in the western mountains appeared to be well planned, with hundreds of fighters from different mountain tribes and battalions converging on their targets in lethal, fast strikes in pickup trucks.

“In months of fighting, this is one of our biggest days,” said Abdul Hadi Mushaikh, a rebel infantry commander, who called the day “pivotal.”

The rebels took three towns — Tekut, Hawamid, Ghezaia — along a road that has long been a stronghold of Gaddafi forces, who used their positions to fire Grad rockets at opposition fighters and keep them bottled up in the mountains.

If the rebels can hold the towns, it puts them about 60 miles from major coastal roads held by Gaddafi forces that supply the capital, Tripoli.

The rebel fighters say they decided to strike based on their own intelligence, as well as aerial photographs supplied by NATO forces, which led them to conclude that Gaddafi was beginning to reinforce troops in the region.

By nightfall, rebels were also fighting in or around three more towns — Tiji, Badr and Jawsh. Rebel spokesmen and ambulance crews said that at least one opposition soldier was killed and a dozen were wounded.

Rebels in Hawamid said they encountered fire from snipers and close-quarter fighting. As they spoke, Grad rockets fired by retreating Gaddafi artillery units continued to land in and around the town.

Outside Tekut, Gaddafi forces headquartered in a roadside house were apparently attacked just as they were drinking their morning coffee. In the aftermath, half-drunk cups lay scattered on a tray beside loaves of bread. The shower stall was still wet, and rooms were piled with rumpled bedding.

The rebels said Gaddafi’s forces included regular soldiers, as well as foreign fighters from central African states. The rebels said they also encountered volunteers recently recruited by Gaddafi.

In a recorded message released on state television Wednesday, Gaddafi warned the western mountain rebels that he was coming for them.

“Surrender, you traitors! The people of Libya are pushing forward — choose to surrender or die,” he shouted.

At a highway intersection outside Tekut, firetrucks sprayed water on smoldering crates of abandoned ammunition. At an electricity generating station, the green flag of the Gaddafi regime flew from the roof.

There was no sign of civilians. Rebel soldiers roared down the roads in pickup trucks whose beds carried machine guns and rocket launchers. All day long, Radio Free Nafusa, a station operated by the rebels, played prayers proclaiming, over and over, “God is great.”

Staff writer Joby Warrick in Washington contributed to this report.