Syrian activists say at least 160 fighters from both sides have been killed in a two-day battle on the outskirts of Damascus.
The BBC reports that it began when rebel units attacked a string of military checkpoints encircling the opposition-held suburbs in an area known as Eastern Ghouta, which has been under siege for more than six months.
Aid workers say that forces loyal to President Bashar al-Assad appeared to be trying to starve out residents - indiscriminately affecting civilians as much as rebel fighters.
Reuters reports that the blockade has cut off rebels' weapons supplies and helped turn the tide of fighting around the capital in Assad's favour.
The battle has also drawn in hundreds of foreign fighters on both sides.
"It is a ferocious fight between the two sides because it's a struggle over our ultimate fate here," said Bara Abdelrahman, a media activist with the rebel Islam Army brigades in the area.
The conflict in Syria has killed more than 100,000 people, according to the United Nations, and is also destabilising Syria's neighbours.
Foreign powers are trying to bring the warring sides together for a peace conference in Switzerland before the end of the year.
On Sunday, Syria's peace envoy Lakhdar Brahimi met members of the opposition to discuss the talks, which many rebel groups have rejected without a clear guarantee that Assad will step down.
Mr Assad's government says it welcomes talks but will not accept any preconditions.
