The top U.S. intelligence chief said on Tuesday the war in Syria had created an "apocalyptic disaster," Reuters reports.
According to the report, National Intelligence Director James Clapper was asked by Senator John McCain during a Senate hearing whether he had seen recently published documents that offer evidence that Syrian government officials committed serious war crimes.
Clapper said he had, and that he believed they were real.
"They're terrible. And when you consider the humanitarian disaster in addition to the 2.5 million refugees, the 6.5 million or 7 million that are internally displaced, the 134,000 plus people that have been killed, it is an apocalyptic disaster," he said, according to Reuters.
McCain, an Arizona Republican, asked if it was Clapper's "professional opinion" that the documents are authentic.
"I believe they are. I have no reason to doubt that, and it would be difficult to suggest, you know, that something of that magnitude could have been fabricated," Clapper told the Senate Armed Services Committee.
The documents in question presented photographic evidence which showed that Syrian government officials committed serious war crimes in the form of systematic torture and killing.
A team of internationally renowned war crimes prosecutors and forensic experts authored the report, which was based on thousands of photographs of dead bodies of alleged detainees killed in Syrian government custody.
The experts who authored the report said the evidence in the photos would stand up in an international criminal tribunal.
The photos have been described as horrific by both the United States and the United Nations.
Clapper said his expectations for Syrian peace negotiations in Geneva are "pretty modest" and called prospects for a long-term political solution to the three-year-long civil war "problematic."
He said his expects a "sort of prolonged stalemate" in which President Bashar Al-Assad's government does not have the strength to hold onto territory it clears, and the opposition has enough external support to keep fighting.
The strength of the Syrian insurgency is now estimated at anywhere from 75,000 fighters to 115,000, organized into more than 1,500 groups of "widely varying political leanings," Clapper said, according to Reuters.
"Somewhere in the neighborhood of between 20,000 and maybe up to a top range of 26,000 we regard as extremists," he said. "And they are disproportionately influential because they are among the most effective fighters on the battlefield."
Among 7,500 foreign fighters from 50 countries, Clapper said, are Al-Qaeda veterans from Afghanistan and Pakistan who aspire to attack Europe, as well as the United States.
U.S. intelligence officials recently expressed their concern over the still small but slowly-increasing presence of American citizens among Islamist rebel groups in Syria.
Out of the estimated 1,200 rebel factions battling Assad, only a handful are either formally linked with Al-Qaeda - such as the Al Nusra Front and the Islamic State of Iraq and the Levant (ISIS) - or inspired by its ideology without having formal links to the group.
Nevertheless, such groups are increasingly gaining influence over other more "moderate" factions.
Estimates released last week said that at least 50 American citizens are fighting in Syria against Assad, and are liable to bring terrorism back to the U.S. once the war is over.
The presence of jihadist rebels in the civil war in Syria has become a major concern for the West, which has reportedly been arming the more moderate rebel groups.
In late January it was reported that light arms supplied by the United States are flowing to "moderate" Syrian rebel factions in the south of the country and that funding for months of further deliveries has been approved by Congress.