Syrian government forces have ambushed rebel fighters in a strategic suburb of the capital Damascus, killing at least 49 people, activists said.
The Syrian Observatory for Human Rights on Tuesday reported that the opposition fighters were killed near Adra, a town which saw heavy fighting when Assad's forces recaptured it from the rebels a few months ago.
The town is located on a route that rebels have been using to smuggle weapons into Damascus.
The latest violence came amid reports that rebels have taken over the village of Khan al-Assal in Aleppo province, the site of an alleged deadly chemical attack last March.
The seizure of the village near the northern city of Aleppo followed three days of intense clashes between rebels and government forces.
The Syrian government and the opposition have accused each other of using chemical weapons in a missile attack that killed at least 25 people in Khan al-Assal, one of the last villages in the western part of Aleppo province that was held by Assad's forces.
On Tuesday, Robert Serry, a UN envoy on the Middle East peace process, said that his organisation had been told of 13 alleged chemical weapons attacks in Syria.
The allegations came as the head of a UN investigation into the use of chemical arms in Syria, Ake Sellstrom, and the UN disarmament chief, Angela Kane, arrived in Beirut.
Deadly air strikes
The two officials were expected to go to Damascus on Wednesday to start talks with Assad's government on securing access to sites in Syria where the arms are said to have been used.
Meanwhile, warplanes and helicopter gunships have reportedly carried out air raids in the town of Tabqa in the northeastern province of Hasakeh, in the central province of Homs and in Aleppo.
At least six people, including women and children, were reportedly killed in an air attack on the northwestern province of Idlib, activists said.
Five others were reported killed when artillery rounds struck Damascus suburb of Douma.
In the south of the capital, troops pounded the Palestinian refugee camp of Yarmuk for the third consecutive day while government forces and rebels were locked in fierce fighting inside the camp, the Observatory reported.
Government forces also shelled neighbourhoods in the south, east and north of Damascus.
The conflict in Syria, which erupted more than two years ago, has left almost 100,000 people killed, according to UN figures.