Secretary of State Rex Tillerson is under pressure to chronicle war crimes committed by Russia and Syria, from a bipartisan group of lawmakers that is pushing for "accountability" at the end of the Syrian civil war.

"We respectfully request that you work to ensure [Syrian President Bashar] Assad, Russia, and Iran are made to answer for the war crimes and crimes against humanity committed in Syria," Sen. Cory Gardner, R-Colo., and 13 other members of the Senate Foreign Relations Committee, wrote to Tillerson on Friday.

The senators renewed their call for designating Assad as a war criminal in light of an Amnesty International report that said between 5,000 and 13,000 people have been tortured and executed in a Syrian prison.

"Sufficient documentation exists to charge Bashar al-Assad with war crimes and crimes against humanity," the senators wrote. "He has lost legitimacy as Syria's leader."

Amnesty International published a report this month on "a hidden, monstrous campaign" to kill civilians "authorized at the highest levels of the Syrian government." The report was based on interviews with 84 people, including 34 men formerly detained at Saydnaya Military Prison and "four prison officials or guards" who worked there.

"The guard would ask everyone to take off all their clothes and go to the bathroom one by one," the report quoted one witness. "As we walked to the bathroom, they would select one of the boys, someone petite or young or fair. They would ask him to stand with his face to the door and close his eyes. They would then ask a bigger prisoner to rape him... No one will admit this happened to them, but it happened so often... Sometimes psychological pain is worse than physical pain, and the people who were forced to do this were never the same again."

The senators also accused Russian forces of committing war crimes "such as the bombing of a humanitarian convoy on September 19, 2016" in support of Assad. They want Tillerson to make the case for the withdrawal of Russian and Iranian forces from Syria as well.

"Russia and Iran's ongoing military operations in support of Assad make Russian and Iranian leaders complicit in Assad's war crimes and crimes against humanity," they wrote. "As you review U.S. policy toward Russia and participate in the Administration's planning to defeat ISIS, Russia's role in the tragic deaths of hundreds of thousands of Syrians must be considered. Russia must join the international community in seeking to hold Assad accountable, stop enabling the slaughter of the Syrian people, and undertake efforts to remove Iran-affiliated fighters from Syria."

That last step would help alleviate a growing worry among U.S. policymakers that, following the defeat of ISIS as a terror-state by American-led coalition forces, Russia and Iran might mobilize to destroy other U.S.-backed forces in the country.

"[Iran will] immediately begin the push to take away all of our influence and try to push us out of there, and probably use their Shia allies to threaten our troops and other personnel on the ground," Sen. Marco Rubio, R-Fla., who signed the letter to Tillerson, predicted in a recent interview with the Washington Examiner.