Washington (CNN) A bipartisan group of US lawmakers sent a letter Thursday to Attorney General William Barr and FBI Director Christopher Wray asking the Department of Justice to investigate allegations of war crimes against renegade Libyan Gen. Khalifa Haftar, who is also an American citizen.

The letter comes one month after President Donald Trump praised Haftar during a phone call as the field marshal's troops continued their offensive against the UN- and US-supported government in Tripoli.

"A United States citizen is directly undermining United States policy in Libya, including US support for a United Nations-led mediation process and the internationally recognized government of Libya. At the same time, Mr. Haftar's forces are alleged to have committed war crimes and inflicted unnecessary suffering and cruelty during the course of military operations," the letter reads.

The request sent to the Justice Department was signed by five House Democrats -- Tom Malinowski of New Jersey, Gerald Connolly of Virginia, David Trone of Maryland, Ted Lieu of California and Colin Allred of Texas -- as well as two Republicans, Joe Wilson of South Carolina and Ann Wagner of Missouri.

Haftar commands the Libyan National Army, which began an offensive to seize Tripoli from the United Nations-backed Government of National Accord in April.

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