The White House said today that Syrian forces under President Bashar al-Assad have used chemical weapons “on a small scale” against the opposition rebels.
A senior aide to President Barack Obama said the US estimated 100-150 people had died in “multiple” attacks.
Ben Rhodes said the US president had decided to provide unspecified “military support” to the opposition.
The White House had previously warned that the US considers the use of such weapons crossing a “red line”.
Ben Rhodes, a deputy national security adviser to Barack Obama, said the US had no “reliable” evidence the opposition had used chemical weapons.
Earlier, the United Nations said the number of those killed in the Syrian conflict had risen to more than 93,000 people.
Ben Rhodes said the president had made the decision to increase assistance, including “military support”, to the opposition’s Supreme Military Council (SMC).
He declined to provide further details, other than to say it would be “different in scope and scale to what we have provided before”.
“The president has been clear that the use of chemical weapons – or the transfer of chemical weapons to terrorist groups – is a red line for the US,” Ben Rhodes said.
“Our intelligence community now has a high confidence assessment that chemical weapons have been used on a small scale by the Assad regime in Syria. The president has said that the use of chemical weapons would change his calculus, and it has.”
Ben Rhodes said US intelligence agencies had concluded Bashar al-Assad’s forces had used chemical weapons, including the nerve agent sarin, based on battlefield reports, “descriptions of physiological symptoms” from alleged victims, and laboratory analysis of samples obtained from alleged victims.
However, the full number killed by chemical weapons was “likely incomplete”, Ben Rhodes said in a conference call with reporters.
“Put simply, the Assad regime should know that its actions have led us to increase the scope and scale of assistance that we provide to the opposition,” he said, including direct support to the SMC.
“These efforts will increase going forward.”
Further actions will be taken “on our own timeline”, Ben Rhodes said.