President Barack Obama has decided to authorise lethal aid to Syrian rebels but US officials are still grappling with what type and how much weaponry to send and how to ensure it stays out of the hands of extremists.
The White House announced it had conclusive evidence that Syrian President Bashar Assad's regime used chemical weapons against opposition forces. Mr Obama has said the use of chemical weapons would cross a "red line," suggesting greater American intervention.
While a small percentage of the 93,000 people reportedly killed in Syria are said to have died from chemical weapons - US intelligence puts the number at 100 to 150 - the White House views the deployment of the deadly agents as a flouting of international norms. Ben Rhodes, Obama's deputy national security adviser, said the multiple chemical weapons attacks gave greater urgency to the situation.
"Suffice it to say this is going to be different in both scope and scale in terms of what we are providing," Mr Rhodes said . But US would make specific determinations "on our own timeline."
The Obama administration could give the rebels a range of weapons, including small arms, assault rifles, shoulder-fired remote-propelled grenades and other anti-tank missiles. The opposition forces could operate most of that equipment without significant training.
Mr Obama's opposition to sending American troops into Syria makes it less likely the US will provide sophisticated arms or anti-aircraft weapons that would require large-scale training. Administration officials are also worried about high-powered weapons ending up in the hands of terrorist groups. Hezbollah fighters are among those backing Assad's armed forces, and al Qaida-linked extremists back the rebellion.
The CIA and special operations trainers are already running some weapons training programmes for the rebels and are expected to take charge of teaching the opposition how to use the weapons the US has agreed to supply, another US official said.
The US has made no decision on operating a no-fly zone over Syria, Mr Rhodes said. The US has so far provided the Syrian rebel army with rations and medical supplies.
Russian President Vladimir Putin is one of Mr Assad's most powerful backers and his foreign affairs adviser said Moscow was not convinced by Washington's claims. Yuri Ushakov told reporters that the information provided by US officials to Russia "didn't look convincing."
The Syrian government dismissed the US claims it used chemical weapons as "full of lies." A statement by the Syrian Foreign Ministry in Damascus said the US was resorting to "cheap tactics" and fabrications to justify Mr Obama's decision to arm the rebels.