The United Nations has ordered all of its non-essential international staff to leave Syria, saying the escalating violence is making it more risky for humanitarian workers to do their jobs.
The UN also plans to reduce some of its field work in the troubled Arab state.
More than 40,000 people have been killed since Syrians began a revolt against president Bashar Assad in March last year. Dozens of aid workers have been caught in the crossfire and killed.
Officials said up to a quarter of the 100 international staff working for several UN agencies could exit by the end of the week.
The UN has also decided to halt field trips into Syria from the capital Damascus by its international staff except in certain emergency cases.
Some UN agencies are relocating staff from the hard-struck city of Aleppo, according to the UN Office for the Co-ordination of Humanitarian Affairs.
"The increasing insecurity is making it more difficult for humanitarian actors to operate and to address the needs of affected people," said Radhouane Nouicer, the UN's regional humanitarian co-ordinator for Syria.
"We are looking at how we can adjust our methods of work so that we continue to reach as many people in need as we can throughout the country."
Mr Nouicer said: "Most of all, we need an end to the unrelenting violence. All calls, from all sides, for parties to honour their obligations to protect civilians in Syria have had little effect, and ordinary people are paying the price."
Eight UN staff have been killed since the start of the 20-month uprising that has turned into a full-blown civil war. Eighteen volunteers with the Syrian Arab Red Crescent, through which most international aid has been channelled, have been killed, including seven while they were on duty.
UN officials said there was no exact limit on the number of international staff who might remain in Syria.
The fighting over the past few weeks in and around Damascus has been the most serious in the capital since July, when rebels captured several neighbourhoods before a swift government counter-offensive swept them out.
Separately, Syria's foreign ministry spokesman has left the country and is heading to the UK, according to Lebanese security officials.
It is not clear if Jihad Makdissi has defected from the Assad regime.
The Lebanese officials said Mr Makdissi left today on a flight from Beirut.
AP