The United Nations is to move about half of 100 foreign staff members out of Syria after a number of mortar shells fell near their hotel in Damascus, damaging the building and a UN vehicle.
“The United Nations Security Management Team has assessed the situation and decided to temporarily reduce the presence of international staff in Damascus due to security conditions," UN spokesman Martin Nesirky said.
"The United Nations remains active and committed to helping the Syrian sides in their search for a political solution," he said.
"U.N. agencies and their partners also remain committed to providing assistance to millions of people in need in Syria."
The founder of the Free Syrian Army rebel group, which is fighting to overthrow Syrian President Bashar al-Assad, was wounded by an explosion in Syria overnight.
Colonel Riad al-Asaad was brought to Turkey for treatment, a Turkish official said.
The colonel lost a leg in the blast but his injuries were not life-threatening.
He established the FSA in 2011 to fight for the overthrow of President Assad and was one of the first senior officers to defect from the Syrian military.
Syrian opposition sources said Col Asaad had been hit by a car bomb in the city of al-Mayadin, south of Deir al-Zor in eastern Syria.
Meanwhile, Syrian rebels have fired dozens of mortar bombs into central Damascus, hitting a high-security area within a kilometre of President Assad's residence, residents and a security source said.
The military retaliated with artillery fire from Mount Qasioun, overlooking the Syrian capital.
It was some of the heaviest fighting in the heart of the city since an uprising against President Assad erupted two years ago.
The security source said mortar rounds had landed in Umayyad Square, a major intersection where military headquarters and state television are located.
Yesterday, mortar bombs hit the car park of the state television building, residents said.
State television reported dozens of casualties at the Opera house, across the street from the headquarters of President Assad's Baath party the Air Force Intelligence building.
The mortar bombs appeared to be coming from rebels who had pushed into the Kfar Souseh district, a few hundred metres from Umayyad Square, but there were no immediate reports of the insurgents trying to advance further.
President Assad's forces have retained control of central Damascus and most other Syrian cities, while losing swathes of territory elsewhere, especially in the north and east.
The United Nations says at least 70,000 people have been killed in the conflict and more than a million Syrians have been forced to flee the country.
Sustained fighting in Damascus could send thousands more into neighbouring states, especially Lebanon, which already hosts 370,000 refugees.