* Saudi Arabia and Qatar already openly arming insurgents
* Britain, France keen to keep radical Islamists in check
* Washington still cautious but Kerry has shifted debate
* No certainty that rebels can topple Assad, even with help
France, Britain and the United States are inching towards providing military aid to Syria's rebels, hoping to beef up more secular forces at the expense of radical Islamists who are gaining ever more prominence in the uprising.
The bitter experience of Afghanistan, where the American arming of anti-Soviet mujahideen forces in the 1980s helped to give rise to the Taliban and al Qaeda, hangs heavy in the air.
And as in Libya, the leaders of Britain and France seem keener than U.S. President Barack Obama to get more involved.
But with Saudi Arabia and Qatar already openly arming the Syrian rebels, Britain and France - both committed with Washington to seeing President Bashar al-Assad leave - want at least to make sure that weapons go to the "right" groups and are not so advanced that they could pose a threat to the West.
They believe they are now in a position to do so - two years after the outbreak of an uprising that has escalated into civil war, with around 70,000 people dead and one million homeless.
"The well-known arguments against arming the rebels - finding a political situation first, not militarising the situation or having weapons falling into the wrong hands - are losing their impact," a senior Western diplomat said.
"We have now identified where the weapons can go and who will get them. The pressure on Bashar is not working and we can't allow one side to continue to get assassinated."
In the last two weeks, Britain and the United States have announced a significant increase in "non-lethal" support to the insurgents. Both have also become much more publicly supportive of efforts by Saudi Arabia and Qatar to arm the rebels.
France said on Thursday it would work with Britain to convince fellow European Union members to relax an embargo on providing arms to Syria. Prime Minister David Cameron said on Tuesday that Britain might be prepared to bypass the embargo.
Backing the rebels is increasingly seen as almost the only leverage foreign powers have. Unless more moderate forces are armed, Islamists with superior equipment and training thanks to Saudi and Qatari support may well grow ever more powerful.
"It may be (that) by doing nothing, the situation gets worse and the level of jihadism gets worse," Cameron told a parliamentary committee this week. "By working with the opposition, by supplying parts of the opposition ... you can at least have influence with your partners."
Ashfon Ostovar, regional analyst at the Centre for Naval Analyses, a U.S.-government funded think-tank that advises the U.S. military among other clients, said: "The bottom line is that they (the rebels) are getting weapons.
"It's a very difficult choice ... but if we don't supply, someone else will."
Many diplomats and analysts fear that the longer Syria's war goes on, the greater the threat of a major Arab state at the heart of Middle East conflicts fragmenting into armed chaos, endangering its neighbours.
British officials say that Britain, and therefore France too, is unlikely to act without the backing of the United States, which has so far been more cautious.
But Washington insiders say the arrival of Secretary of State John Kerry - who as chairman of the Senate Foreign Relations Committee argued last year that Washington should arm rebel forces - subtly shifted that dynamic.
"In almost every institution - the State Department, Pentagon, CIA, as well as the think-tanks, the newspapers - people think the U.S. should be doing more," says Ari Ratner, a former Obama Administration State Department political appointee and now a fellow at the Truman National Security Project.
"The problem is that no one can really agree what should be done, because there are no easy options."
Ultimately, few believe that medical supplies, radios, body armour or even armoured, four-wheel drive vehicles will significantly tilt the war against Assad. Nor have the arms supplied by Gulf countries produced a breakthrough.
The German weekly Der Spiegel and Britain's Guardian said U.S., British and French instructors are training Syrian anti-government fighters in Jordan with the help of Jordanian intelligence services, aiming to build a dozen units totalling 10,000 fighters, but excluding radical Islamists.
European security officials have played down those reports. But, as well as groaning under the strain of 300,000 Syrian refugees, Jordan can see Islamist militancy rising on its doorstep and is more worried than Saudi Arabia or Qatar.
Late last year, the United States listed Jabhat al-Nusra, one of the most successful Islamist rebel groups fighting in Syria, as a banned terrorist organisation.
"The Jordanian intelligence services want to prevent Salafists (radical Islamists) crossing from their own country into Syria and then returning later to stir up trouble in Jordan itself," one of the organisers of the training told Der Spiegel.
Some say that letting the Syrian opposition have basic anti-tank weaponry is relatively low-risk - particularly systems capable of damaging Assad's elderly Soviet tank fleet but much less effective against modern U.S. or Israeli models.
But French President Francois Hollande said last year that he favoured giving opposition fighters anti-aircraft weapons to defend "liberated areas".
If these are what the rebels need most, to fight off Assad's fighter-bombers and helicopter gunships, they are also what Washington worries about most - fearing they could fall into the hands of militants who would turn them on civilian aircraft. The rebels are believed to have seized some from government stockpiles, but there is little appetite for providing more.
For now, anyway, no Western country has gone as far as explicitly offering weapons.
But last week, both Kerry and British Foreign Secretary William Hague warned that if Assad continued to dig in, further options were already being discussed.
"It is something that has to be under discussion if you look at the direction of travel," said another Western official.
"Direct military action is not being discussed directly but it's always something that could be on the table under certain circumstances."
Germany, the other main European power, remains opposed to arming the insurgents directly. However, Foreign Minister Guido Westerwelle said it was now "necessary to show more flexibility and to understand that we have of course to support the ... opposition in a responsible way".
In the end, the decisive voice will be that of U.S. President Barack Obama.
Several senior policymakers told Congress that Obama last year rejected suggestions to arm the rebels backed by key officials of the time: Secretary of State Hillary Clinton, Defense Secretary Leon Panetta and CIA chief David Petraeus.
"He (Obama) feels strongly that the immediate answer is not to empower more killing," Kerry told U.S. National Public Radio.
"It is rather to try to say to President Assad: 'There is a solution'. Now if Assad doesn't want that, then he's asking, obviously, for yet another ratcheting-up."