WASHINGTON — The United States accused the Syrian government on Monday of using a crematory to hide mass murders at a prison where thousands are believed to have been summarily executed in the nation’s civil war.

The assertion, based on newly declassified satellite photographs of the prison, significantly expanded the accusations the United States has leveled 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 offenses against humanity,” said Nikki R. Haley, the American ambassador to the United Nations.

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