For Syria's divided and beleaguered rebels, the creeping realisation that there will not be a decisive Western military intervention on their behalf is a huge psychological blow.

President Bashar Assad's regime has gained strength, largely because the world community is concerned that if he is toppled the result may be an Islamist Syria in the grip of al-Qaida.

The immediate result has been an uptick in fighting between moderate and jihadi rebels.

The long-term outcome is likely to be a prolonged war of attrition that continues the slow destruction of Syria as a coherent state and further fans the flames of sectarian hatred and extremism in a turbulent Middle East.

Only two weeks ago, the Obama administration appeared poised to launch a US military strike against the Syrian regime in response to the chemical weapons attack it says was launched by Assad's forces, killing hundreds of civilians in opposition-controlled areas near Damascus.

President Barack Obama reversed course after an ambitious agreement between the US and Russia calling for an inventory of Syria's chemical weapons programme within a week, and for all its components to be removed from the country or destroyed by mid-2014.

Assad immediately signed on, and on Friday sent the Organization for the Prohibition of Chemical Weapons an "initial declaration" outlining Syria's weapons programme.

The agreement abruptly reshuffled the cards, baffling opposition forces who had held out hope that US-led strikes would help tip the scales in the country's deadly stalemate. The conflict, now in its third year, has killed more than 100,000 people and uprooted millions of people from their homes.

Obama warned in August 2012 that any deployment of the Syrian government's chemical weapons stockpiles was a "red line" that would bring harsh consequences. Now, the realisation that even a sarin attack the US says killed more than 1,400 people would not trigger military action has left rebels feeling bitter and powerless, and is likely to strengthen extremist trends in rebel ranks.

While the US and Russia, a staunch Assad ally, have stepped up efforts to hold a peace conference for Syria, the Russian deal makes it less likely the Syrian sides will agree to talk. Opposition forces say the agreement effectively legitimizes Assad's regime, at least until mid-2014 when chemical weapons stockpiles are supposed to have been destroyed. Presidential elections are due around the same time, and Assad has suggested he may run again.

Opposition forces say that by agreeing to relinquish his chemical weapons stockpile, Assad successfully removed the threat of US military action while giving up very little in exchange. Unlike the regime's warplanes, which have pushed back rebel advances and pummelled opposition-held territory, chemical weapons are not seen as crucial to the regime's survival or military strategy.

The focus on diplomacy, which is likely to drag out the war, bolsters the jihadi narrative that the West is not interested in a rebel victory after all.