Syrian forces loyal to President Bashar al-Assad seized a strategic town east of Damascus on Wednesday, breaking a critical weapons supply route for the rebels, activists and fighters said, Reuters reported.
Rebels have held several suburbs ringing the southern and eastern parts Damascus for months, but they have been struggling to maintain their positions against a ground offensive backed by fierce army shelling and air strikes in recent weeks.
"The disaster has struck, the army entered Otaiba. The regime has managed to turn off the weapons tap," a fighter from the town told Reuters via Skype.
"The price of a bullet will go from 50 Syrian pounds to 1,000 Syrian pounds ($10) now, but we must pay and retake it. It's the main if not the only route."
Rebels said they pulled out of Otaiba, a gateway to the eastern rural suburbs of Damascus known as al-Ghouta, in the early hours after more than 37 days of fighting in which they accused the government of using chemical weapons against them twice.
The government has denied using chemical weapons and accused rebels in turn of firing them in Aleppo.
Rebels used Otaiba for eight months as their main supply route to Damascus for weapons brought in from the Jordanian border, where Saudi Arabia and other private donors are believed to be sending in arms.
Government forces pushed in with tanks and soldiers.
"Now all the villages will start falling one after another, the battle in Eastern Ghouta will be a war of attrition," another fighter in the area said, speaking by Skype.
More than two years into their struggle to end four decades of Assad family rule, the rebels remain divided by struggles over ideology and fighting for power
Rebels fighting in Otaiba said they sent a distress call to brigades in other parts of Ghouta but it went unanswered by other units with whom they compete for influence and weapons.
"To all mujahedeen (holy warriors): If Otaiba falls, the whole of Eastern Ghouta will fall ... come and help ," part of the message sent to fighters said.
The army appears to have been advancing on fronts across Syria in recent weeks, even in northern provinces where rebels seized large swathes of territory.