WASHINGTON • The United States has accused the Syrian government of using a crematorium to hide mass murders at a prison where thousands are believed to have been summarily executed during the nation's civil war.

The assertion on Monday, based on newly declassified satellite photographs of the prison, significantly expanded the accusations the US has levelled at President Bashar al-Assad of Syria, with some officials appearing to draw a parallel to Nazi Germany.

"The attempt to cover up mass murders in the Assad crematorium is reminiscent of the 20th century's worst offences against humanity," said Mrs Nikki Haley, the US Ambassador to the United Nations.

Mr Assad's forces and their allies have been widely accused by human rights groups and others of killing prisoners and burying their bodies in mass graves during the war, now in its seventh year.

But the Trump administration went further on Monday, contending that the government was systematically incinerating the corpses of executed inmates at the Sednaya prison complex north of Damascus to destroy evidence that could be used to prosecute war crimes.

"We now believe that the Syrian regime has installed a crematorium in the Sednaya prison complex, which could dispose of detainees' remains with little evidence," said Mr Stuart E. Jones, the Acting Assistant Secretary of State for Near Eastern Affairs, at a news conference in Washington. "At this point, we are talking about this evidence and bringing it forward to the international community, which we hope will put pressure on the regime to change its behaviour."

Mr Jones acknowledged that the satellite photographs, taken over the past four years, were not definitive. But in a photograph from 2015, he said, the buildings were covered in snow - except for one, suggesting a significant internal heat source. "That would be consistent with a crematorium," he said.

Officials said a discharge stack and architectural elements thought to be a firewall and air intake were also suggestive of a place to burn bodies.

The UN is scheduled to begin another round of Syria peace talks in Geneva yesterday, and the timing of the accusations seemed intended to pressure Russia, Mr Assad's principal foreign ally, into backing away from him.

"The rest of the world recognises the horrors of the Syrian regime," said Mrs Haley. "It is time for Russia to join us." There was no immediate comment from the Russian or Syrian governments. Mr Assad has repeatedly denied committing crimes in the war, which he has framed as a struggle against terrorism fomented by his Western and Arab enemies.

Human rights groups said they were surprised by the Trump administration's assertions, in part because some of the satellite photographs have existed for years and are not conclusive.

NYTIMES, AGENCE FRANCE-PRESSE