Military commander of the Libyan rebels fighting to topple Col Muammar Gaddafi has been killed, the rebel National Transitional Council says.
NTC head Mustafa Abdul-Jalil said Gen Abdel Fattah Younes was killed by assailants, and the head of the group responsible had been arrested.
He said Gen Younes was summoned for questioning about military operations, but never made it to the meeting.
Reports said Gen Younes was suspected of ties to pro-Gaddafi forces.
“With all sadness, I inform you of the passing of Abdel Fattah Younes, the commander-in-chief of our rebel forces,” Mr Jalil announced late on Thursday. “The person who carried out the assassination was captured.”
Mr Jalil did not elaborate on the identity or motivations of the assailants. It is not clear where the attack took place.
Correspondents say Mr Younes was not trusted by all the rebels because of his previous role in cracking down on dissidents.
Two aides to Gen Younes, Col Muhammad Khamis and Nasir al-Madhkur, were also killed in the attack, Mr Jalil said, adding that none of the bodies had been recovered.
There will be three days of mourning in their honour, he said.
Gen Younes is a former Libyan interior minister who defected to the rebel side in February.
He was also part of the group that helped bring Col Gaddafi to power in 1969.
Some unconfirmed reports said Gen Younes and two aides had been arrested earlier on Thursday near Libya’s eastern front.
Shortly after the announcement of Gen Younes’ death, gunmen entered the grounds of the hotel in the eastern city of Benghazi where Mr Jalil was speaking, reportedly firing into the air before being convinced to leave.
Earlier on Thursday, rebels said they had seized the strategically important town of Ghazaya near the Tunisian border, after heavy fighting with Col Gaddafi’s forces.
They reportedly took control of several other towns or villages in the area.
The rebels are struggling to break a military deadlock five months into the uprising against Col Gaddafi’s rule.
Rebels control most of eastern Libya from their base in Benghazi and the western port city of Misrata, while Col Gaddafi retains much of the west, including the capital, Tripoli.
Late on Thursday there were reports of explosions shaking the centre of Tripoli, as state TV reported that planes were flying over the Libyan capital.
Nato, acting under a UN mandate authorising military action for the protection of civilians, has carried out regular airstrikes in the Tripoli area.
Meanwhile, the South African ambassador to the UN, Baso Sangqu, warned that supporters of the rebels were in danger of violating UN sanctions.
His comments came a day after Britain granted the rebels diplomatic recognition and said it would unblock £91 million ($149m) in frozen Libyan oil assets for the rebels.
“We have noted the calls for Gaddafi must go,” Mr Sangqu said. “We maintain that such statements do not bring us any closer to a political solution.”
The british media reporting from the UN said that the growing trend to grant diplomatic recognition to the Libyan rebels is facing opposition on the Security Council, and that moves to back the rebels will further polarise Council members.
Portugal has become the latest of about 30 countries to have recognised the NTC.
There are a lot of questions about exactly what happened on Thursday and what the sequence of events was. We’re told that Gen Younes had been due to appear before a panel of rebel judges to answer a number of questions on a military matter. We’re not entirely sure what that was.
In a conflict like this where there are not many facts and one side of the country is fighting another, we tend to get an awful lot of rumours and suggestions.
For what it’s worth, some of the ones we’re hearing is that there was a worry among the rebels that the military campaign was not going very well. They wanted to question the general.
Some of the speculation we’re hearing in Tripoli, coming from Col Gaddafi’s side, is that the general was simply not trusted by the rebels.
Abdel Fattah Younes
•Helped Libyan leader Col Muammar Gaddafi take power in the 1969 coup that ousted King Idris
•Close advisor to the Libyan leader for four decades, rising to the post of general and training Col Gaddafi’s special forces
•Appointed interior minister
•Quit the government on 22 February 2011 and defected to the rebels – one of the earliest such moves by a senior official
•Appointed as the opposition’s military chief in April, but faced mistrust due to his past ties to Col Gaddafi.