"The regime has cut off the last main road for rebels leading out of Eastern Ghouta," the main opposition stronghold in Damascus province, said Syrian Observatory for Human Rights chief Rami Abdel Rahman.
He said army units had taken over almost all of the village of Maydaa, which lies along a road running east which rebels use to bring food and reinforcements into besieged neighbourhoods.
Syria's official news agency SANA quoted a military official as saying the army had taken full control of Maydaa.
"A large number of terrorists are thought to be dead," SANA reported, adding that army units "shut the last route for terrorists" that ran east.
Eastern Ghouta has been under a devastating government siege for nearly two years in an attempt to break the rebel hold over the area.
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Abdel Rahman told AFP rebels could still rely on a few small roads that lead out of Eastern Ghouta but said they were "very dangerous".
He said there were clashes underway in Maydaa between Syrian regime forces and Jaysh al-Islam, the most powerful rebel group operating in the area.
Jaysh al-Islam spokesman Islam Alloush said Syrian troops tried to overrun Maydaa but were "ambushed" by the rebels.
"Clashes are still underway but if the army succeeds in taking Maydaa they could use it as a launchpad to storm Eastern Ghouta," Alloush told AFP by telephone from Turkey.
The rebel bastion of Eastern Ghouta has been subjected to massive regime bombardments for months.
Syria's four-year conflict began with anti-regime protests in mid-March 2011 and spiralled into a bloody war after a harsh government crackdown on demonstrators.
More than 210,000 people have been killed in the conflict, according to the Observatory.