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. Two aides to Gen Younes, Col Muhammad Khamis and Nasir al-Madhkur, were also killed in the attack, Mr Jalil said, adding that there would be three days of mourning in their honour.

The exact circumstances of the killings were unclear, and Mr Jalil did not say directly that the assailants were allied with Col Gaddafi.

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.