The Trump administration accused the Syrian government Monday of carrying out mass killings of thousands of prisoners and burning the bodies in a large crematorium outside the capital. It also stepped up criticism of Iran and Russia for supporting the Syrian government.

The allegation came as U.S. President Donald Trump is weighing options in Syria, where the U.S. attacked a government airbase last month in response to an alleged chemical weapons attack on civilians.

But Trump hasn't outlined a larger strategy for ending the Arab country's civil war or ushering Syrian President Bashar al-Assad out of power. These questions were sure to arise in his meeting at the White House with the United Arab Emirates' crown prince Monday, a day before Turkey's president arrives.

The State Department said it believed that about 50 detainees a day are being hanged at Saydnaya military prison, about 45 minutes north of Damascus. Many of the bodies, it said, are then burned in the crematorium.

"We believe that the building of a crematorium is an effort to cover up the extent of mass murders taking place," said Stuart Jones, the top U.S. diplomat for the Middle East, in accusing the Syrian government of sinking "to a new level of depravity."

Acting Assistant Secretary for Near Eastern Affairs Stuart Jones briefs the press on the situation in <a href="https://twitter.com/hashtag/Syria?src=hash">#Syria</a>. <a href="https://t.co/atXOkJp1BG">pic.twitter.com/atXOkJp1BG</a> —@StateDept

The department released commercial satellite photographs showing what it described as a building in the prison complex that was modified to support the crematorium. The photographs, taken over the course of several years, beginning in 2013, do not definitely prove the building is a crematorium, but they show construction consistent with such use.

In presenting the photographs, Jones called on Russia and Iran to press Assad's government to establish a credible ceasefire with Syrian rebel groups and begin negotiations on a political settlement.

"We are appalled by the atrocities that have been carried out by the Syrian regime, and these atrocities have been carried out seemingly with the unconditional support from Russia and Iran," Jones said.

Satellite images — taken in August 2013, left, and April 2015 — show what the U.S. State Department describes as a building in a prison complex that was modified to support a crematorium. (State Department/DigitalGlobe via AP)

U.S. urges Russia to rein in Assad

Nikki Haley, the U.S. ambassador to the UN, said in a statement on Monday that "Assad bears the largest responsibility for his own brutality," but added Russia needs to do more.

"The rest of the world recognizes the horrors of the Syrian regime," said Haley. "It is time for Russia to join us."

State Department spokesperson Heather Nauert said Secretary of State Rex Tillerson had been "firm and clear" in a meeting with Russian Foreign Minister Sergey Lavrov last week that "Russia holds tremendous influence over Bashar al-Assad."

A fighter with the Syrian rebels looks toward the northern town of Tabqa, Syria, in April. (Syrian Democratic Forces/Associated Press)

A main point of that meeting "was telling Russia to use its power to rein in the regime," she said. "Simply put, the killing, the devastation has gone on for far too long in Syria."

The war has killed as many as 400,000 people since 2011. It has contributed to Europe's worst refugee crisis since the Second World War and enabled ISIS to emerge as a global terrorism threat.

Trump travels to the Middle East later this week on his first official foreign trip.