The Trump administration has accused the Syrian Government of carrying out mass killings of thousands of prisoners and burning the dead bodies to hide evidence.

Key points: Satellite images presented as evidence of crematorium

Satellite images presented as evidence of crematorium US alleges bodies could be disposed with little trace

US alleges bodies could be disposed with little trace Amnesty didn't uncover evidence of a crematorium during its investigation

Amnesty didn't uncover evidence of a crematorium during its investigation But it says there's no doubt building used as a torture and execution centre

The US State Department released newly declassified photographs showing what it alleges is a building at a large military prison outside the capital Damascus that has been modified to support a crematorium where thousands of bodies have been burnt.

Stuart Jones, acting Assistant Secretary of State for Near Eastern Affairs, said US officials believe the crematorium could be used to dispose of bodies at a prison where they believe Syrian President Bashar al-Assad's Government authorised the mass hangings of thousands of inmates during Syria's six-year-old civil war.

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"Credible sources have believed that many of the bodies have been disposed in mass graves," Mr Jones said.

During the briefing, he showed aerial images of what he said was a crematorium.

"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," Mr Jones said.

The revelations echoed a February report by Amnesty International that said an average of 20 to 50 people were hanged each week at the Sednaya military prison, north of Damascus.

Between 5,000 and 13,000 people were executed at Sednaya in the four years since a popular uprising descended into war, it said.

But researchers from Amnesty confirmed to the ABC they did not uncover evidence of a crematorium during their investigation.

Their year-long investigation involved first-hand interviews with 84 witnesses that included former Saydnaya guards, officials and detainees.

Amnesty said while it could not comment on the US claims of a crematorium, it had no doubts regarding the use of Saydnaya as a torture and execution centre that killed thousands.

As recently as last week, the NGO received concerning reports of movement of prisoners in the notorious "red building" at Saydnaya.

The NGO continues its call for the Syrian Government to allow independent observers into Saydnaya and all other Syrian detention facilities.

A satellite view of part of the Sednaya prison complex near Damascus from a video briefing provided by the US State Department. ( Reuters: Department of State )

Syria has not responded to the latest allegations from the US, but it has in the past strongly denied any abuse at the jail, saying that all executions followed "due process".

Madhar Sharabjty, a Syrian refugee in Turkey whose brothers were detained by the Assad regime in 2011, said the news was extremely worrisome.

"Today we heard the statement from the US State Department concerning the fate of the Syrian prisoners," he said.

"This news had made us very sad … now we lost hope, because now the fate of my brothers and all the detainees is totally unknown."

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US again hits out at Russia, Iran backing of Assad

Mr Jones also said he was not optimistic about a Russia-brokered deal to set up "de-escalation zones" inside Syria.

The deal was reached with support from Iran and Turkey during ceasefire talks in the Kazakh capital of Astana earlier this month. Mr Jones attended the talks.

"In light of the failures of the past ceasefire agreements, we have reason to be sceptical," Mr Jones said.

Mr Jones said Mr Assad's Government had carried out air strikes, chemical attacks, extrajudicial killings, starvation, and other measures to target civilians and its opponents.

He criticised Russia and Iran for maintaining their support for Mr Assad despite those tactics.

"These atrocities have been carried out seemingly with the unconditional support from Russia and Iran," Mr Jones said.

"The [Assad] regime must stop all attacks on civilian and opposition forces. And Russia must bear responsibility to ensure regime compliance."

He did not say what measures the United States might take if Russia does not change its stance.

Tensions between the United States and Russia heightened after President Donald Trump ordered a cruise missile strike in April against a Syrian air base that the United States said had been used to launch a poison gas attack on civilians.

Mr Jones said he had not yet presented the evidence to Russian officials.

He said he hoped Russia would help pressure the Assad Government.

ABC/wires