The main opposition group in Syria - the Free Syrian Army (FSA) - is welcoming President Barack Obama's promise to take on and defeat Islamic State extremists.
A spokesperson for the army has urged the Americans to begin training and equipping opposition fighters as soon as possible.
Speaking from neighbouring Lebanon, the BBC's Paul Wood reported that the reaction of the FSA is "cautiously welcoming" of American help.
"There's a measure of skepticism, too. They feel they've been promised help many times before, only to get next to nothing. To the Syrian regime, however, helping the FSA amounts to arming one group of terrorists and declaring war on another, to quote the official news agency."
Ten Arab countries including Saudi Arabia have agreed to help the United States in its fight against Islamic State extremists.
The announcement followed a meeting with the US Secretary of State, John Kerry, in Jeddah.
Mr Kerry told the BBC the detail of who does what is still being worked out but participants recognised the danger posed by the group and fully backed President Barack Obama's strategy to combat it.
He downplayed the fact that Turkey did not sign the communique.
"Turkey's very engaged and very involved. I'm going to be visiting there and I have full expectation as we go forward that we'll work through whatever issues exist."
More Islamic State fighters than previously feared - CIA
The American Central Intelligence Agency says the Islamic State may have up to 31,000 fighters in Iraq and Syria - three times as many as previously feared.
The CIA previously believed IS had about 10,000 fighters.
The agency's spokesperson Ryan Trapani said the new estimate was based on a review of intelligence reports from May to August.
He said it reflected an increase in members because of stronger recruitment by IS since June following battlefield successes and the declaration of a caliphate, greater battlefield activity and additional intelligence.
The jihadist group has seized vast swathes of Iraq and beheaded several hostages in recent months, leading to air strikes by the United States.
The revision comes a day after Mr Obama outlined a plan to "degrade and destroy" the jihadist group.