Libya's ruling National Transitional Council said Saturday its choice for the post of army chief of staff was "irreversible" despite objections raised by some influential ex-rebel groups.

"The decision of the NTC to appoint Colonel Yussef al-Mangush is final and irreversible," NTC member Mokhtar al-Jadal told AFP in the eastern city of Benghazi, which first rose against toppled dictator Moamer Kadhafi last February.

Jadal, who is also a member of NTC's media committee, said Mangush was chosen by a majority of NTC members.

Mangush himself told AFP that he had already joined office as head of the new Libyan army, after his nomination on Tuesday, but was still to be appointed officially.

Two powerful groups of former rebels, the Coalition of Libyan Thwars (revolutionaries) and Cyrenaica Military Council, which helped oust Kadhafi, have opposed the appointment of Mangush.

"We reject anybody who is not among the list of six candidates proposed by us to the NTC," Behlool Assid, a founder of the Coalition of Libyan Thwars, told AFP on Wednesday.

The coalition represents powerful factions of former rebels from major Libyan cities such as Benghazi, Misrata and Zintan. These militias are heavily armed, including with artillery guns and tanks.

Cyrenaica Military Council, which represents fighters from eastern Libya, even went as far as to name its own alternative candidate to head the new national army.

But some other groups of fighters have expressed their support for Mangush.

A colonel in the former army of Kadhafi, Mangush joined the rebellion against the veteran dictator.

During the conflict, he was arrested in the oil town of Brega in April by Kadhafi's forces and freed in late August following the fall of Tripoli.

The post of chief of staff had been vacant since the murder last July of General Abdel Fatah Yunis, who commanded the rebels in eastern Libya against Kadhafi's diehards.

Yunis had been expected to head the new Libyan army when it was formed.