Elsewhere Thursday, a car bomb exploded outside a research center belonging to the Ministry of Industry in the area of Soumariya near Damascus, killing four people and wounding several others, a government official said. The official who spoke on condition of anonymity because he was not authorized to talk to reporters.
Three people were wounded when mortar shells hit two residential neighborhoods of Damascus, the state news agency SANA reported.
In the northern province of Aleppo, a Syrian surgeon working for an international aid group was killed. Doctors Without Borders said the 28-year-old surgeon, Muhammad Abyad, died in an attack. Abyad, whose body was found Tuesday, had been working in an Aleppo hospital run by the group.
The conflict started in March 2011 as largely peaceful demonstrations against Assad's rule. It turned into a civil war after opposition supporters took up arms to fight a brutal government crackdown on dissent. Two years of fighting have led to a stalemate, with the rebels controlling much of the countryside in the north, east and south, and the regime holding most urban centers in the west, where the majority of Syrians live.
More than 100,000 people have been killed, with nearly 7 million people uprooted from their homes. U.N. officials estimate that 5 million have been displaced inside the country while another 2 million have fled to neighboring countries. The total amounts to nearly a third of Syria's population, which was 23 million before the fighting began.
U.N. humanitarian chief Valerie Amos met with Syrian government officials in the capital, lobbying for access to civilians trapped in areas where fighting has raged.
After meeting with the president of the Syrian Arab Red Crescent, Amos told the AP that she is "extremely concerned that the situation on the ground is becoming worse."
An alleged chemical attack near Damascus in August has brought the U.S. to the brink of launching punitive airstrikes after the Obama administration concluded that Assad's forces were responsible.
Obama has been lobbying for international and domestic support for punishing the regime, which the U.S. says fired rockets loaded with the nerve agent sarin on rebel-held areas near Damascus before dawn Aug. 21, killing hundreds of people.
Obama has called chemical weapons use a "red line." Top administration officials have argued before the U.S. Senate and around the world that Assad would take inaction by Washington as a license for further brutality against his people.
So far, however, Obama has won little international backing for action. Among major allies, only France has offered publicly to join the U.S. in a strike.
At the Group of 20 economic summit in St. Petersburg, Obama will confront Syria's closest supporter, Russia, as well as foreign leaders skeptical of his call for an international military intervention in Syria.
Moscow and Washington have sharply disagreed over ways to end the bloodshed, with Russia protecting the Assad regime from punitive actions in the United Nations. The U.S. has backed the opposition and repeatedly has called on Assad to step down. He has refused and the U.S. has been supporting the rebels with non-lethal aid and by training some rebel units in neighboring Jordan.
Russia's Foreign Ministry said Syrian Foreign Minster Walid al-Moallem will travel to Moscow on Monday for a meeting with his Russian counterpart.
European Union President Herman Van Rompuy urged U.N. investigators to release information as soon as possible about the purported chemical weapons attack Aug. 21 so that the international community can decide how to respond.
SANA said Syrian Parliament speaker Jihad Laham sent a letter to the President of the European parliament, Martin Schulz, inviting members to visit Syria to follow up on the outcome of the U.N. investigation and help develop terms of reference for any subsequent investigations.