Up to 25 of about 100 foreign staff could leave this week, it said, adding that 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.
"We can confirm that the United Nations in Syria will pull out non-essential international personnel with immediate effect," UN Secretary-General Ban Ki-moon's spokesman Martin Nesirky told reporters.
"The UN will also suspend its missions within the country until further notice," he said in New York.
In all, the world body deploys more than 1,000 national and international staff in Syria, but movement and communications have become more difficult due to intensified fighting near the capital and a 48-hour Internet blackout last week, the UN Office for the Coordination of Humanitarian Affairs (OCHA) said.
Some UN agencies are relocating staff from the northern town of Aleppo, which rebels are battling to control, it said.
"My understanding is that international staff will be going to Jordan," a UN source in Geneva told Reuters.
Damascus had been considered "quite safe" until last week when the main airport was shut down and flights into Syria cancelled after several attacks by rebels, OCHA said.
"The situation is significantly changing," said Sabir Mughal, the UN's chief security adviser in Syria. "There is an increased risk for humanitarians as a result of indiscriminate shooting or clashes between the parties."
MORTARS AND SHOOT-OUTS
For two thirds of the month of November, UN security considered many main routes in the country unsafe for travel or there was not enough information to send staff on field trips with confidence, OCHA said on its news service www.irinnews.org .
"Mortars have landed and shoot-outs have taken place just steps away from UN offices; shelling can be heard on an hourly basis both from their offices and places of residence," it said.
Eight UN staff have been killed along with seven volunteers of the Syrian Arab Red Crescent during the 20-month-old conflict which has claimed more than 40,000 lives.
The UN's World Food Programme (WFP) has been supplying food rations to up to 1.5 million people in Syria, many of them forced to flee their homes due to fighting.
The UN Children's Fund (UNICEF) is providing assistance to families and helping to repair water and sanitation systems, while the World Health Organization (WHO) is delivering medical aid to hospitals and clinics.
The UN refugee agency has supplied winter blankets and clothing to some of the more than 1.2 million displaced within Syria, including in the flashpoint city of Homs last week, and to the 465,000 refugees registered in neighbouring countries.
The International Committee of the Red Cross (ICRC) is continuing operations "per usual", Geneva spokesman Alexis Heeb said. The independent humanitarian agency - whose aid workers do not travel in armoured vehicles - stopped using the airport road months ago, considering it unsafe, he said.
"We're going ahead with distributions as usual, including a new water project," Heeb told Reuters on Monday evening.