Two powerful groups of former rebels who helped oust dictator Moamer Kadhafi rejected on Wednesday the government's choice of a new army chief, raising fresh security concerns in the war-torn country.

The Coalition of Libyan Thwars (revolutionaries) and Cyrenaica Military Council -- representing militias from several parts of Libya -- said they do not accept the choice of Yussef al-Mangush as chief of staff as declared by officials on Tuesday.

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

"We reject anybody who is not among the list of six candidates proposed by us to the NTC (National Transitional Council)," Behlool Assid, a founder of the Coalition of Libyan Thwars, told AFP on the sidelines of a news conference.

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

Security concerns stem from the fact that any dispute over who should head the army will delay forming the overall force itself and integrating former rebels into it.

Several officers in the former army have also regularly criticised the NTC for moving slowly on appointing a new chief of staff, saying the delay had held back the formation of a new army and the integration of ex-rebels.

Forming a new army is seen as a key step towards disarming militias in Libya, especially in the capital Tripoli, where a gunbattle between ex-rebels on Tuesday killed four fighters.

The four were killed when gunmen from Misrata clashed with a militia faction from a central Tripoli neighbourhood in broad daylight, trading anti-aircraft and heavy machinegun fire.

The NTC and the interim government of Prime Minister Abdel Rahim al-Kib, who has said that the issue of disarming these former rebels is "complex," intend to integrate tens of thousands of these fighters into the security forces.

On Wednesday, the planning ministry proposed that former rebels who join the security forces of the interior and defence ministries will receive a minimum monthly salary of 600 Libyan dinars ($500 dollars), the government website reported.

The ministry also proposed that those fighters who want to return to civilian life receive aid for studies in Libya or abroad.

Assid told reporters his group was disappointed as NTC chief Mustafa Abdel Jalil himself on December 21 had urged the fighters to put forward names for the the post of national army chief.

"The thwars had agreed to support the candidate who is selected from the list we proposed ... We feel that the procedure with which Mangush has been appointed is illegal," Assid told reporters, adding that "selecting the army chief is not so easy."

"The NTC is handling the appointment of such a sensitive post very negatively. It is not doing proper evaluations."

The six candidates proposed by the former rebels were largely unknown.

On Tuesday, two NTC members, Abdelrazzak al-Aradi and Fathi Baaja, told AFP that Mangush, a former colonel in Kadhafi's military and who fought against the former dictator during the conflict, had been chosen to head the army.

Mangush is currently deputy defence minister in Kib's interim government.

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

For its part, Cyrenaica Military Council named Salah Salem al-Obeidi as its man to head the national army, the council's political coordinator Ezzidine al-Waqwaq told AFP. Obeidi, who was described only as a "general," was not among the six names proposed to the NTC.

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. Obeidi, proposed by Cyrenaica Military Council, hails from the same tribe as of Yunis.