TRIPOLI - Libyan ex-fighters furious over a decision to halt a cash rewards scheme opened fire on Tuesday against the headquarters of the interim government, but no injuries were reported, sources said.

"The headquarters of the council of ministers was attacked by armed groups who fired their weapons," read a statement issued by the office of the prime minister, making no mention of casualties.

The gunmen were "protesting the decision of the National Transitional Council to stop financial grants allocated to the revolutionaries," who waged war against the regime of slain leader Moamer Gathafi last year.

A source close to the prime minister said no one was hurt in the attack.

"The attack was short -- just a scare tactic," the source said.

The interim government slammed the incident as an assault against "the sovereignty and prestige of the state" and rejected the "language of threats and blackmail."

The ruling National Transitional Council announced on Monday a pause in the payment of bonuses to former rebels due to the widespread phenomenon of fraud which has cost millions of Libyan dinars.

The Tuesday statement reiterated that the freeze would remain in place until a probe into the matter was completed.

Militiamen angered by non-payment have recently held small protests in front of the headquarters of the interim authorities in Tripoli and raised checkpoints blocking traffic in some neighbourhoods of the capital.

In the eastern city of Benghazi, also on Tuesday, a convoy carrying the United Nations' envoy to Libya was attacked but no one was injured.

Acts of violence including the desecration of Christian and Sufi graves have fuelled fears that extremist elements are gaining momentum in post-Gathafi Libya.

Analysts and rights group have repeatedly warned that the interim government must disarm militias who pose one of the biggest challenges to the north African nation's transition to democracy.