President Barack Obama has said the US has evidence of the use of chemical weapons in Syria and that his administration might have to "rethink the range of options" available to Washington with regard to its strategy in the country.
In a rare Tuesday press conference at the White House, Obama said the Pentagon and other security agencies had been told to make preparations for possible action if the "red line" of chemical weapon use had indeed been crossed.
"What we now have is evidence that chemical weapons have been used inside of Syria, but we don't know how they were used, when they were used, who used them," he said. "We don't have chain of custody that establishes what exactly happened."
The Assad government has been accused of using weapons loaded with nerve agents against targets in civilian areas during the country's two-year-long conflict.
"I've got to make sure I've got the facts. That's what the American people would expect ... If I can establish in a way that not only the United States but also the international community feel confident in the use of chemical weapons by the Assad regime, then that is a game changer," Obama said.
In-depth coverage of escalating violence across Syria
Obama did not rule out eventual US military action, but remained cautious.
"By game changer, I mean we would have to rethink the range of options that are available to us. We are invested in trying to bring back a solution inside of Syria," he told reporters.
"Obviously, there are options that are available to me that are on the shelf right now that we have not deployed, and that's a spectrum of options."
Obama said as early as 2012, he had instructed the Pentagon, military and other intelligence officials "to prepare for me what options might be available", but would not elaborate on the details of those options.
Military action 'unlikely'
Al Jazeera's Patty Culhane, reporting from Washington, said public sentiment in the United States would make direct military action highly unlikely.
Referring to a recent poll, our correspondent said "the vast majority of Americans do not want to see the US have any military role in Syria".
Further complicating the likelihood of direct US action in the Syrian conflict was Obama's reference to the confidence of the international community, Culhane said.
"Satisfying the international community would present a much different set of requirements."
Meanwhile, the Syrian regime again accused rebels of using chemical weapons.
Syria's UN ambassador alleged on Tuesday that opposition fighters had used "chemical material" during an attack near the city of Idlib.
Ambassador Bashar Jaafari told reporters the incident was an attempt to make it look as though government forces had used chemical arms.
Jaafari said "terrorist groups" had "spread seemingly the contents of plastic bags containing a kind of powder which must be most probably a chemical material".
The ambassador said many people were affected by the "heinous and irresponsible act" that was an attempt to "implicate the Syrian government on a false basis".
The Syrian government is refusing to let UN experts into the country to investigate whether chemical weapons have been used in the conflict, in which the UN says more than 70,000 people have been killed.
The government wants any investigation limited to its allegations that the opposition used chemical arms near the city of Aleppo last month. The United Nations and western countries want other allegations investigated.