Earlier this week, the Russian foreign minister also warned against a repeat of the "Iraqi scenario" in which claims that Saddam Hussein's government possessed so-called weapons of mass destruction were the basis of the U.S.-led invasion. He also said that international investigators were asking "too much" by demanding access to all facilities in Syria and to have the right to interview any Syrians.
In a letter sent to lawmakers before U.S. Defense Secretary Chuck Hagel announced there was evidence sarin has been used in Syria, the White House said that intelligence analysts have concluded "with varying degrees of confidence that the Syrian regime has used chemical weapons on a small scale in Syria, specifically the chemical agent sarin."
The White House cautioned that the "chain of custody" of the chemicals was not clear and that intelligence analysts could not confirm the circumstances under which the sarin was used, including the role of al-Assad's regime.
British Prime Minister David Cameron said that he supported Obama and that the use of chemical weapons should constitute a "red line," the UK Press Association reported.
But if a red line has been crossed, Cameron was less clear on what the next steps should be.
Asked if the development could result in sending troops into Syria, Cameron said he didn't want to see that.
"But I think we can step up the pressure on the regime, work with our partners, work with the opposition in order to bring about the right outcome," he said, according to the Press Association. "The question is how do we step up the pressure. And, in my view, what we need to do -- and we're doing some of this already -- is shape that opposition, work with them, train them, mentor them, help them, so that we put the pressure on the regime and so what we can bring this to an end."
The Syrian government has been battling a rebellion for more than two years, bringing international condemnation of the regime and pleas for greater international assistance.
The United Nations estimated in February that more than 70,000 people had died since the conflict began.
The opposition Local Coordination Committees in Syria reported that 139 people, including 16 women and 14 children, had been killed across the country on Friday. Twenty-nine of those deaths were in and around Damascus, while 27 were in Homs province.
Members of the rebel Free Syrian Army clashed with government forces in at least 115 places around Syria on Friday, during which the opposition group reported 235 bombing attacks -- including from warplanes and surface-to-surface missiles.