Even those who want an end to the regime worry about who is joining the fight. A young British Islamist seems to confirm their fears. Justin Vela reports from Aleppo.
When Syria's rebel cause has a British accent
ALEPPO, Syria // Wounded Syrian rebels being carried in and out of crowded hospitals are hardly an unusual sight. But when one young fighter passing through the hospital doors says, "Excuse me, guys", in perfect British English, bystanders take notice.
As he walks towards the lorry that will return him to the front line, Abu Yacoub refuses to reveal his age or real name. A convert to Islam five years ago, he accompanied another rebel fighter, an Iraqi named Hassan, to the hospital for treatment to a leg wound.
Then comes the part of his biography that worries even those governments keen to see Bashar Al Assad removed from power but who shudder at the implications of the armed opposition becoming a magnet for foreign, especially Islamist, fighters. "Abu Yacoub" is British, from east London, and plans to be in Syria "until I die".
"Every soul should taste death," he says.
The number of foreign militants fighting to overthrow Mr Al Assad is not known, though unconfirmed news reports indicate the insurgent ranks have been augmented by rebels from Libya, Saudi Arabia, Pakistan and Bangladesh, as well as Britain, Iraq and the southern Russian republic of Chechnya.
If their numbers grow, analysts say, foreign fighters could further transform the anti-Assad struggle into a sectarian fight between Sunnis and Alawites, the branch of Shiism to which the president and his closest associates belong. It is regarded by some Sunnis as heretical.
For foreign rebels such as Abu Yacoub and Hassan, Syria's uprising "takes on a much more sectarian tone", says Andrew Tabler of the Washington Institute for Near East Policy. "They want to knock out this essentially Shiite dictator."
Crossing borders to fight a despot, infidel or foreign occupier is not a new phenomenon in the region.
Mohammad Ali Najar, a Syrian doctor in a medical clinic in Aleppo, says he and other volunteers from Aleppo were joined by a German Muslim and some Palestinians when they travelled to Iraq in 2003 to fight US-led coalition troops.
Calling themselves the People's Army, they bribed their way through a border crossing to enter Iraq, he says. "I went there to fight as an Arab and a Muslim."
Having taken up arms against the Americans in Iraq and seen three friends killed, Mr Najar might now be expected to distance himself from any rebel pleas for help from Washington and other western capitals. He is pragmatic, though.
"America should support the people. I have no problem to be friends with Americans if they are with the right side."
Abu Ibrahim, a commander in Jabhat Al Nusra, a Syrian Islamist militant group said to have ties to Al Qaeda, agrees.
"We will accept any help from any source, but without interference in our national affairs," Mr Ibrahim says.
While western governments are concerned about the influence of Islamist fighters on the opposition to Mr Assad, some experts have accused the regime itself of allowing Al Qaeda to carry out attacks in Syria in hopes of discrediting the insurgency.
"I am quite sure the regime has had a hand in this, at least in the beginning," says Thomas Pierret, an expert on Syria and political Islam at the University of Edinburgh.
Nevertheless, Syria is not fertile ground for Islamist extremists, Mr Pierret contends. It is dominated by mainstream Islamism such as the Muslim Brotherhood and similar ideologies, which are less strident than extremists such Al Qaeda, he says.
That puts the burden on Islamist fighters from abroad to "prove to Syrians that they are doing something about Syria".