Fears that Russian mercenaries, unofficially backed by Moscow, are tipping the scales in Libya in favour of the military leader Khalifa Haftar appears to have prompted the US to issue a strong warning to the general to pull back.

The US has been accused of taking little interest in the eight months of fighting on the outskirts of Tripoli that began when Haftar’s Libyan National Army mounted an assault on Tripoli, provoking a fierce counter-offensive by militia forces supporting the Government of National Accord (GNA).

Recent clashes have left more than 1,000 people dead, but divisions in the international community have allowed the carnage to continue with little effective UN intervention.

Reports that Russian mercenaries employed by the Wagner group are tipping the balance in favour of Haftar has raised fears in Washington that Libya could be lost to Russia in the same way as Syria. The unofficial Russian forces are seen to be testing the ground to help Haftar win control of the oil-rich country.

So far Haftar has been dependent on military help from Egypt and the United Arab Emirates. The GNA has had practical backing from Turkey, in defiance of a UN country-wide arms embargo.

There are US special forces in Libya, but they are dedicated to fighting Islamic State rather than siding with the government.

The warning to Haftar came after US Department of State officials in Washington met the Libyan interior minister, Fathi Bashagha, from the UN-backed, Tripoli-based GNA.

A US official said on Thursday: “The Government of National Accord’s delegation expressed grave concerns regarding the security situation and its effect on the civilian population.

“The United States calls on the Libyan National Army to end its offensive on Tripoli. This will facilitate further US-Libya cooperation to prevent undue foreign interference, reinforce legitimate state authority, and address the issues underlying the conflict. The US delegation, representing a number of US government agencies, underscored support for Libya’s sovereignty and territorial integrity in the face of Russia’s attempts to exploit the conflict against the will of the Libyan people.”

Previously the US has appeared neutral by calling for a ceasefire rather than urging one side to withdraw. It remains to be seen if this new political support is not countermanded in a tweet by the president, Donald Trump, and whether the statement leads to practical US pressure either on Haftar or his sponsoring allies to pull back.

In a skilful lobbying pitch, the GNA appears to have persuaded the US that Russia wants to install an authoritarian government in Libya that will brush aside plans prepared by the UN special envoy, Ghassan Salamé, for elections. “The Russians have intervened to pour fuel on the fire and enhance the crisis rather than finding a solution,” Bashagha said, criticising the deployment of the Russian paramilitary group.

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“The proof is their deployment of Wagner to Libya. They had sent it to Syria before, and central Africa, and wherever Wagner goes, destruction happens,” he said. Russia denies the existence of the group in Libya.

The Haftar offensive has stalled, even though dangerous night airstrikes with drones and also F16s from the Emirates and Egypt continue. Targets include civilian airports and installations increasingly closer to Tripoli’s centre. More than 300,000 people have been displaced.