The death toll given by the Obama administration for an alleged Syrian chemical weapons attack is far higher than confirmed counts of two key allies and a main activist group, which said it was shocked by the U.S. figure.
In pressing Congress to authorize a military strike against Syria, the administration has asserted that the government of President Bashar Assad killed 1,429 people, including at least 426 children, in an Aug. 21 attack on the suburbs of Damascus.
But Britain and France have cited far lower numbers of confirmed deaths, raising questions about the intelligence the White House is using to make its case to launch missile strikes against Syria.
U.S. officials say they can't disclose how they derived their figure without compromising intelligence, but they say it is based on a variety of sources and they stand by it.
British intelligence organizations said last week that they believed at least 350 people had been killed. French intelligence said Monday that it had confirmed at least 281 deaths through open-source videos, although its experts had created models that were consistent with as many as 1,500 deaths.
The Syrian Observatory for Human Rights, generally regarded as one of the most reliable sources of information on casualty figures in Syria, says it has confirmed 502 deaths, including 80 children and 137 women. Rami Abdul-Rahman, a Syrian expatriate who runs the organization from his home in Britain, said he was shocked by the White House's count.
"I don't know where this number came from," Abdul-Rahman said in a phone interview.
He said some Syrian opposition groups disseminate propaganda and exaggerated death tolls in an attempt to sway American politicians.
"The U.S. took this high number from one part of the Syrian opposition that is supported by the U.S. government," Abdul-Rahman said. "We don't trust them."
U.S. intelligence officials said they didn't base their assessment on reports from any single opposition group.
Adding to the confusion, a day after Secretary of State John F. Kerry said that "the United States government now knows that at least 1,429 Syrians were killed in this attack," President Obama on Saturday only referred to "well over 1,000" fatalities.
Kerry didn't cite any casualty figures in his testimony Tuesday before the Senate Foreign Relations Committee. And according to an official familiar with a formal intelligence assessment sent to Congress, the document did not include those figures.
The casualty figures are important because the administration is resting its case for military action in part on the scale of the attack. A senior administration official said a previous use of chemical weapons by Assad's military caused "100 to 150" casualties, which prompted the Obama administration to offer lethal aid to Syrian rebels, but not to contemplate direct military action.
"This is a much larger scale of use," the official said Friday. "It's indiscriminate."
U.S. intelligence officials said Tuesday that foreign intelligence agencies use different methods and resources and often arrive at different casualty counts. A British official in Washington said the British estimate was lower because it was released a few days before the American one.
Shawn Turner, a spokesman for the U.S. director of national intelligence, said, "The information that led to our understanding of the scale of the attack came from a number of sources, including international and Syrian medical personnel, videos, witness accounts and social media reports.
"We've said from the beginning that our estimates were preliminary and that we expect to update the number as we obtain more information," he said.
A senior administration official, who would not be quoted by name discussing classified intelligence, said that Obama was not signaling any retreat from the higher figure and that officials believed the death toll could even go beyond 1,429.
A second official said, "The president has no intelligence reason to walk back from that number."
The precision of the U.S. figure, given the initial confusion surrounding the attack and the often contradictory reports of the violence in Syria, also raised questions among some observers.
Anthony Cordesman, an analyst at the Center for Strategic and International Studies, wrote Monday that the U.S. estimate was "absurdly over-precise" and that Obama's language created "a mix of contradictions over the most basic facts."
Syria's political and military opposition is severely fragmented. The U.S. figure more closely matches reports by pro-opposition organizations such as the Local Coordination Committees and the United Revolutionary Medical Office in eastern Ghouta, a Damascus suburb hit by the alleged chemical attack.
The latter group said it had documented at least 1,302 deaths, about two-thirds of which were women and children. That figure was cited by the U.S.-based Syrian Support Group, which has a federal license to funnel aid to Syrian rebels.