U.S. Defense Secretary Chuck Hagel on Thursday said the U.S. intelligence community believes the Syrian government has used sarin gas on a small scale against rebels trying to overthrow President Bashar Assad.
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Speaking to reporters in Abu Dhabi, Hagel said the White House has informed members of Congress that, within the last day, U.S. intelligence believes with "some degree of varying confidence" that Assad's regime has used chemical weapons - specifically sarin gas.
"It violates every convention of warfare," he said.
The White House added that such assessments were not enough and that "credible and corroborated" facts were needed.
"Given the stakes involved, and what we have learned from our own recent experiences, intelligence assessments alone are not sufficient - only credible and corroborated facts that provide us with some degree of certainty will guide our decision-making," Miguel Rodriguez, White House director of the office of legislative affairs, said in a letter to lawmakers.
Britain's Foreign Office said on Thursday it had information showing chemical weapon use in Syria, and called on Assad to cooperate with international bodies to prove he had not sanctioned their use.
"We have limited but persuasive information from various sources showing chemical weapon use in Syria, including sarin. This is extremely concerning. Use of chemical weapons is a war crime," a Foreign Office spokesman said in a statement.
Syrian opposition figures are urging the U.S. to act on President Barack Obama's own public assertions that Syria's use of chemical weapons would cross a "red line."
Ahmad Ramadan, a member of the Syrian National Coalition opposition group's executive body, called the U.S. assertion an "important step" that should be followed by actual measures taken by the U.N. Security Council and Friends of Syria group.
Ramadan told The Associated Press by phone from Istanbul that the U.S. has a "moral duty" to act to prevent President Bashar Assad from using more chemical weapons.
Hagel indicated on Wednesday that he was caught by surprise when Israeli officials publicly revealed their assessment that Syria has used chemical weapons in its civil war.
On Tuesday, a senior Israeli military intelligence official publically announced an assessment of chemical weapons use by the Syrian government, but the next day Hagel said he did not have any American intelligence confirming such a claim.
A second senior Israeli military officer, speaking on the condition of anonymity to discuss intelligence matters, said on Tuesday that chemical weapons appear to have been used in five cases. He said dozens of people were killed in the attacks when a sarin-type chemical was dispersed.
The Israeli announcement of chemical weapons use in Syria came a day after Hagel visited Israel. He said that Minister of Defense Moshe Ya'alon did not alert him to the military intelligence assessment on Syria when they met in Tel Aviv.
The Obama administration has said Syrian government use of chemical weapons would be a "game changer," implying that it might, if confirmed, compel the U.S. to get more directly involved in the civil conflict.