Fighting has intensified around Aleppo as government forces seek to keep control of an airport. The main opposition group has said it is boycotting talks in protest at a lack of global condemnation for Damascus.
Fighting raged around Syria's largest city Aleppo on Saturday, a day after a deadly rocket attack on the city by government forces.
The airport, which is currently closed, is a long-standing target of the rebels. Rami Abdul-Rahman, director of the Syrian Observatory for Human Rights, a opposition-linked activist group based in Britain, said the fighting was centered on a stretch of highway linking it with the city.
A military source told the AFP news agency that reinforcements had been sent to the city's airport, as government forces seek to regain the initiative in the broader battle for the country.
Earlier, the main opposition said it would not attend forthcoming talks set to take place in Italy, Russia and the United States.
The National Coalition of Syrian Revolutionary and Opposition Forces cited the failure of world powers to adequately condemn the government of Syrian President Bashar Assad.
"Due to this shameful international position, the coalition has decided to suspend its participation in the Friends of Syria Conference of international powers due in Rome on Thursday and talks in Russia and the US," a coalition statement said.
The grouping has recently offered to hold talks with government officials who do not have "blood of their hands."
The decision to boycott the talks came a day after dozens were killed in a missile attack by government troops on a residential area in the northern city of Aleppo. Three missiles were reported to have killed at least 29 people.
A suicide car bombing close to the ruling Baath Party headquarters in central Damascus killed 53 civilians on Thursday. Opposition activists put the death toll at 61, which would make Thursday the deadliest day since the uprising, in which an estimated 70,000 people have died, began two years ago.
rc/dr (dpa, AFP, AP, Reuters)