The decision was made after a number of mortar shells fell near their hotel in Damascus, damaging the building and a UN vehicle.
UN spokesman Martin Nesirky said on Monday another 800 local staff had been asked to work from home until further notice.
Syrian rebels lobbed mortar rounds into central Damascus on Monday, killing at least two people and drawing a fierce army response as bombardments shook the capital, Reuters reports.
The two-year conflict began as peaceful protests that turned violent when Syrian President Bashar al-Assad tried to crush the revolt. The United Nations says more than 70,000 people have been killed and nearly 1.2 million have fled the violence.
Mr Nesirky said there would still enough staff in country to continue and increase the range of work, particularly to reach people with food aid.
The United Nations said on Thursday it would investigate Syria's allegations that rebel forces used chemical weapons in an attack near Aleppo, but Western countries are seeking a probe of all claims concerning the use of such banned arms.
US and European officials say there is no evidence of a chemical weapons attack. If one is confirmed, it would be the first use of such weapons in the Syrian conflict.
