The United Nations is suspending aid operations in Syria and withdrawing all non-essential international staff due to the worsening security situation.
Up to 25 of about 100 foreign staff could leave this week, it said, adding more armoured vehicles were needed after attacks in recent weeks on humanitarian aid convoys and the hijacking of goods or vehicles.
Some convoys had been caught in crossfire between Syrian government and rebel forces, including an incident near the airport in which two staff were injured, it said.
In all, the world body deploys more than 1,000 national and international staff in Syria, Reuters reports.
The UN Office for the Coordination of Humanitarian Affairs (OCHA) said movement and communications have become more difficult due to intensified fighting near the capital and a 48-hour internet blackout last week.
Some UN agencies are relocating staff from the northern town of Aleppo, which rebels are battling to control.
Rebels have begun to advance more quickly after months of slow sieges to cut off army routes and supplies.
In the past few weeks, they seized several military bases, and they are now using anti-aircraft weapons to attack the military helicopters and fighter jets that bombarded their positions with impunity until now.
Media reports citing European and US officials said Syria's chemical weapons had been moved and could be prepared for use in response.
US Secretary of State Hillary Clinton warned that Washington would take action against any such escalation, but Syria said on Monday it would not under any circumstances use chemical weapons against its own people
"Syria has stressed repeatedly that it will not use these types of weapons, if they were available, under any circumstances against its people," the foreign ministry said.