Last Updated 4:59 p.m. ET
The head of the Libyan rebel's armed forces and two of his aides were killed by gunmen Thursday, the head of the rebel leadership said.
The death of Abdel-Fattah Younis was announced at a press conference in the de facto rebel capital, Benghazi, by the head of the rebels' National Transitional Council, Mustafa Abdul-Jalil. He told reporters that rebel security had arrested the head of the group behind the killing.
Rebel security had arrested Younis and two of his aides early Thursday from their operations room near the rebels' eastern front.
Security officials said at the time that Younis was to be questioned about suspicions his family still had ties to Muammar Qadaffi's regime.
Younis was Qadaffi's interior minister before defecting to the rebels early in the uprising, which began in February.
Abdel-Jalil said that Younis had been summoned for questioning regarding "a military matter." He said Younis and his two aides were shot before they arrived for questioning.
Abdel-Jalil called Younis "one of the heroes of the 17th of February revolution," a name marking the date of early protests against Qadaffi's regime.
While he criticized Qadaffi for seeking to break the unity of rebel forces, he did not say directly that Younis' killers were associated with the regime. Instead, he issued a stiff warning about "armed groups" in rebel-held cities, saying they needed to join the fight against Qadaffi or risk being arrested by security forces.
Younis defected to the rebel side in February, but rumors have spread that he has engaged in secret talks with the Qaddafi regime.
A witness told Reuters that this morning Younis' home in Benghazi was strongly guarded by soldiers, who had blocked the street and were not letting anyone in,
Meanwhile, hundreds of rebels in western Libya launched a broad offensive against government forces Thursday, seizing three small towns and advancing on others to secure a major supply route near the Tunisian border, rebel spokesmen said.
Four rebel fighters were killed and several wounded in what the spokesmen described as the biggest push in the area since the start of Libya's civil war five months ago.
They said rebels captured 18 government soldiers, as well as weapons and ammunition.
Libya's civil war has been largely deadlocked, despite shifting front lines, and rebels have been unable to score a major breakthrough on the battle field. Rebels control Libya's eastern third and most of the Nafusa mountains in the west, as well as the western port city of Misrata. Libyan leader Muammar Qadaffi, entrenched in the capital of Tripoli, is clinging to the rest of the territory.
In Thursday's fighting, hundreds of rebels descended from their stronghold in the Nafusa mountains at dawn and advanced in several areas along a stretch of more than 60 miles, said rebel spokesmen Badees Fessato and Abdel-Salam Othman. Opposition forces drove government troops out of the small towns of Jawsh, Ghezaya and Takut, Othman said.
Rebels were also advancing toward the town of Badr and two other areas.
Jawsh is about 95 miles east of the Tunisian border and located along a main road between Tunisia and Tripoli. Ghezaya is closer to the Tunisian border, and had served as a base for shelling attacks by Qadaffi's forces on the rebel-held border town of Nalut.
One of the objectives of the push is to drive back pro-Qadaffi forces from the rebels' main supply route between the Tunisian border and the Nafusa mountains, the spokesmen said. Rebels seized control of the Tunisian-Libyan border closest to the Nafusa area in April, but Qadaffi's forces have repeatedly tried to retake it and have shelled the supply road.
"The goal is to liberate the area and secure the road from the border to all of the liberated regions," Othman said.
Thursday's offensive came a day after Britain recognized Libya's main opposition group, the National Transitional Council, as Libya's sole representative, dealing another blow to the isolated Qadaffi regime.
However, the rebels appear to be increasingly divided over what would be an acceptable way of ending the civil war.
France, Britain and the U.S. have indicated they would accept an arrangement in which Qadaffi retires, but is allowed to stay in Libya, and the rebel chief, Mustafa Abdul-Jalil, has also suggested he would consider the idea.
But the U.K. organizer for the rebel council, Guma El-Gamaty, said Thursday that the plan is unworkable and shouldn't even be considered.
With the possibility of a political deal seemingly off the table, fighting was bound to continue.
Abdul-Jalil said this week that the rebels would not lay down their weapons during the Muslim holy month of Ramadan, which begins next week and is a time of increased religious observance.
Qadaffi has also signaled defiance in audio messages in recent days. In a message released on state TV late Wednesday, Qadaffi demanded that the western mountain rebels give up their weapons.
"Surrender, you traitors! The people of Libya are pushing forward (to the western mountains) choose to surrender or die," Qadaffi shouted.
He said that without relying on NATO bombings, the rebels would not have been able to hold the mountains. NATO, acting under a U.N. mandate to protect civilians, has been bombing regime-linked targets since March 31.