Foreign ministers of the "Friends of Syria" group meet on Saturday for crucial talks on arming the rebels trying to topple the Damascus regime, with loyalist forces making key gains on the battlefield.
On the eve of the meeting in the Qatari capital, main armed opposition group the Free Syrian Army (FSA) told AFP it had new weapons that could turn the tables on President Bashar al-Assad's regime.
"We've received quantities of new types of weapons, including some that we asked for and that we believe will change the course of the battle on the ground," FSA media spokesman Louay Muqdad said.
"We have begun distributing them on the front lines, they will be in the hands of professional officers and FSA fighters," he said, without saying where the weapons came from.
Senior opposition figure Burhan Ghalioun said the FSA had recently received "sophisticated weapons", including "an anti-aircraft defence system".
The reported boost to the rebel arsenal comes after Washington vowed to increase support for the insurgency after declaring that Assad had defied warnings not to use chemical weapons -- an accusation denied by Damascus.
But Syria's armed opposition wants more sophisticated weapons and has urged its friends in the West and Arab states to impose a no-fly zone over areas it controls.
The CIA and US special operations forces have been training Syrian rebels for months, since long before President Barack Obama announced plans to arm the opposition, the Los Angeles Times reported Friday.
Training for rebel forces covers the use of anti-tank and anti-aircraft weapons and has been carried out at bases in Jordan and Turkey since late last year, the newspaper reported, citing unnamed US officials and rebel commanders.
Officials in Washington said Friday that the US military has expanded its presence in Jordan to 1,000 troops, in a show of force amid the raging civil war in neighboring Syria.
The United States is concerned about a possible spillover of violence from Syria to its southern neighbor Jordan, a key US ally and one of only two Arab states to have signed a peace treaty with Israel.
Syrian rebel appeals for more aid follow a string of military gains by loyalist troops backed up by Lebanon's Iran-backed Hezbollah group.
They have retaken Qusayr in the central province of Homs near Lebanon and are trying to recapture rebel-held areas of the key northern city of Aleppo.
US Secretary of State John Kerry will attend the Doha talks with the foreign ministers of Britain, Egypt, France, Germany, Italy, Jordan, Qatar, Saudi Arabia, Turkey and the United Arab Emirates.
Washington has given no details about the kind of military support it could offer the rebels, and President Barack Obama is cautious about becoming embroiled in an increasingly sectarian conflict.
"The goal of the meeting is to be very concrete about the importance of every kind of assistance that's coming from the London 11 countries (Friends of Syria)... being fully coordinated and going through only the Syrian opposition coalition," a US official said on Friday.
The official called the Qatar meeting critical as the opposition Syrian National Coalition was examining its leadership amid concerns in Washington that fighters battling Assad lack cohesion and direction.
"This is all in support of energising, re-energising, the Syrian opposition coalition leadership to work to select its leadership," the official said on condition of anonymity.
French Foreign Minister Laurent Fabius said the meeting would address how to coordinate Western aid.
"We will try in Doha to sum up the situation on the ground and to see how we can aid the opposition coalition and arrive at a political solution," he said.
France and Britain have pushed for arming the rebels but underscored that this must be done responsibly to avoid the kind of anarchy that followed the fall of Libyan dictator Moamer Kadhafi.
Nearly two years after Kadhafi was toppled and killed, Libya is awash with arms and the scene of frequent deadly violence including attacks on Western targets, much of it blamed on Islamist radicals.
Backers of the Syrian rebels fear that weaponry they provide could fall into the hands of radical groups such as the Al-Qaeda-allied Al-Nusra Front.
Energy powerhouses Qatar and Saudi Arabia are the insurgency's main two providers of aid in the Arab world.
The Doha talks also come after a summit of G8 leaders called for a peace conference on Syria and agreed to push for a transitional government in Damascus, despite deep divisions with Syrian ally Russia.
Opposition figure Ghalioun said on Friday that he expected agreement in Doha on a "practical and coordinated plan to stop counter-attacks by the regime".