Human Rights Watch warned Sunday of the use of cluster bombs over the past four months in the Libyan conflict between forces of the North African country's rival governments.

"There is credible evidence of the use of banned cluster bombs in at least two locations in Libya since December 2014," HRW said in a statement released in Beirut.

The watchdog, on the basis of telephone interviews with witnesses and photographic evidence, said remnants of RBK-250 PTAB 2.5M cluster bombs were found at Bin Jawad, east of Tripoli, in February and in nearby Sirte in March.

"The good condition of the paint on the bomb casings and lack of extensive weathering indicated that the remnants had not been exposed to the environment for long and were from a recent attack," it said.

"It is not possible to determine responsibility on the basis of available evidence," said HRW of what would be the first reported use of cluster bombs since their deployment by the regime of slain dictator Moamer Kadhafi in Libya's 2011 revolution.

The Libyan air force of the internationally-recognised government has acknowledged that it carried out air raids on the two locations but denied the use of cluster bombs.

"While the air strikes continue every day against militias, the Libyan army has access to only traditional, heavy munitions, such as what was used during the Second World War," air force chief Brigadier General Sagr al-Jerouchi told HRW.

"We have no cluster munitions," he said.

HRW urged Libyan authorities to sign up to a 2008 Convention on Cluster Munitions (CCM) banning their use.

"The use of cluster munitions in populated areas, such as Sirte, violates the laws of war due to the indiscriminate nature of the weapon," said Steve Goose, the New York-based group's arms director.

"The new evidence of cluster munitions use in Libya is highly disturbing," he said.

"Most nations have banned these weapons due to their inherently indiscriminate nature and the unacceptable harm they pose to civilians," he said.

"The internationally recognised government in Libya and parties to the conflict should urgently secure and destroy any stocks of cluster munitions."

Libya has been teeming with weapons since its revolution, with rival militias and administrations battling for power. The oil-rich country has two parliaments and two governments.