An empty soda bottle, crushed charcoal and cola-soaked gauze -- Syrian rebels are scrambling to protect against chemical attacks by regime forces by cobbling together DIY gas masks from household items.

US President Barack Obama has renewed a warning to Damascus that the use of chemical arms would cross a "red line" as evidence emerged the deadly nerve agent sarin may have been used against the rebels.

Now, fear of a chemical attack looms large in the mountainous province of Latakia -- the heartland of President Bashar al-Assad's minority Alawite sect -- as Syria's civil war stretches into its third year.

The courtyard of one Free Syrian Army base in the province bustled with activity as rebels took a crash course in how to cobble together a gas mask from household items.

Abu Tarek, 72, is no physicist but fellow fighters from the FSA's Al-Ezz bin Abdul Salam brigade put aside their Kalashnikovs and listened attentively.

First, he cut the top off a large soda bottle, boring a hole for air in the cap, heaping a few spoons of ground charcoal onto a piece of cola-soaked gauze.

He then pushed the gauze into the bottle, and covered it with wet cotton, fixing it in place with cardboard. The mask was then fitted snugly over his face with a piece of elastic.

"It's very simple," Tarek said at the base, a whitewashed house in a sprawling village hemmed between the green slopes of the Jabal al-Turkman and Kurdish mountains.

Tarek, in olive-green fatigues, acknowledged his masks only offer limited protection, failing for instance to guard against corrosive gases that attack the skin.

But they were better than a wet towel and could allow time for an escape from poisonous fumes or advanced nerve agents within minutes of an attack, he said.

Tarek struggled to explain how his crude device works, but said he had picked up the recipe from military service and from information found on the Internet.

With lack of international support, the rebels are desperately banking on makeshift masks -- which he said will also be taught to civilians.

The rebels have heard that regime forces have vacated five Alawite-dominated front line villages on their side in the past week and that Assad's army has been distributing gas masks -- real ones -- to its troops.

Such information is impossible to corroborate independently, as government-held areas are off-limits to foreign journalists, but the rumours have triggered a sense of foreboding among the rebels.

"Why are they vacating their front line villages?" asked brigade commander Abu Basir, combing his beard with his fingers. "It's a great sign that something big is coming."

In the distance the war raged on: government helicopters buzzed overhead, dropping bombs on mountain villages.

Raids like these have become the lynchpin of regime forces' strategy, which gives them an advantage over the rebels who have no air power.

It has been difficult adapting to this new kind of warfare -- rebels no longer gather in huge numbers, not even to pray in mosques, and make sure their hardware is concealed under leafy trees.

The biggest fear among some rebels is that one day, the helicopters will drop a chemical bomb without warning.

Emerging allegations of likely chemical attacks by Assad's regime have stirred international outrage, piling pressure on Obama for a military intervention.

"Should (the) evidence be confirmed as conclusive and intervention of some type be supported by allies, the Obama administration will need to respond... to this latest and most egregious provocation by the Syrian regime," said Mona Yacoubian, a senior adviser for the Middle East at the Washington-based Stimson Center.

"The allegations... certainly appear to cross a red line," she told AFP.

But this "red line" stance evokes amusement among the rebels.

Abu Tarek says his feelings are best summed up by a cartoon by Ali Farzat, a renowned Syrian satirist, in which Assad crosses numerous US-demarcated red lines drawn in the sand to step on the final one that says "chemical weapons".

"When he (Assad) used planes to bomb his own people, he crossed a red line. When he bombed cities, towns and neighbourhoods, he crossed a red line. When he began using scud missiles on civilians, he crossed a red line," he said.

Developments surrounding chemical weapons in Syria
Nicosia (AFP) April 26, 2013 - Below is an account of the main developments surrounding accusations of the use of chemical weapons in Syria, which has been wracked by armed conflict for the past two years:

--2012--

- July 23: The Syrian regime acknowledges for the first time that it has chemical weapons and threatens to use them in case of a Western military intervention, but never against its own people.

- August 20: US President Barack Obama warns Syria of "enormous consequences" if it resorts to chemical weapons, and says the United States would regard any recourse by Damascus to its deadly arsenal as crossing a "red line" which would result in US action.

- December 3: NATO and the United States warn Syria against using its chemical weapons, as a US official says that Syria has begun mixing chemicals that could be used to make sarin, a deadly nerve agent.

- December 24: Russian Foreign Minister Sergei Lavrov says it would be "political suicide" for the government of Syrian President Bashar al-Assad if it used chemical weapons against the armed opposition, but that "I do not believe Syria would use chemical weapons".

--2013--

- January 30: the Israeli air force bombards a site of ground to air missiles and an adjacent military complex near Damascus suspected of holding chemical agents, with Israel saying it feared the transfer of weapons to the Lebanese Shiite movement Hezbollah, according to a US official.

- March 19: The government and rebel forces accuse each other of using chemical weapons for the first time, but the United States says it has seen "no evidence" of such action.

- March 20: Obama warns the Damascus regime it would be held accountable for any use of chemical weapons against civilians in Syria, which would be a "grave and tragic mistake" and a "game-changer."

- March 21: the United Nations says it will open an investigation into an alleged chemical weapons attack in Syria.

- April 8: UN Secretary General Ban Ki-moon says the UN inspection team is in Cyprus and ready to deploy. Syria rejects the mission as proposed by Ban.

- April 25: The White House says that Syria has likely used chemical weapons against rebel forces on a "small scale," during the conflict, while emphasizing US spy agencies are still not 100 percent sure.

UN leader Ban renews an "urgent call" for Syria to let inspectors into the country.

- April 26: British Prime Minister David Cameron says that growing evidence of the use of chemical weapons by the regime is "extremely serious". Israel urges the US to "take control of Syria's chemical weapons arsenal".

Syria's opposition urges the UN Security Council to take immediate action.

Obama is awaiting a "definitive judgement" on whether the Syrian regime used chemical weapons against rebels before taking action, the White House says.