Last week, the international airport in Damascus closed as fighting moved closer to the Syrian capital. Internet and mobile-phone networks were reported down as well.
Radhouane Nouicer, the regional humanitarian coordinator for the United Nations, told the world body's humanitarian news agency IRIN that security needed to be reassessed because of violations to international humanitarian law.
"The security situation has become extremely difficult, including in Damascus," he said.
Sabir Mughal, the top U.N. security adviser, said humanitarian workers are at significant risk in Syria.
The U.N. Supervision Mission in Syria suspended work in the country last summer because of armed conflict. Its mandate was ended in August because of continued violence.
Since conflict began in early 2011, eight U.N. staff members were killed along with a number of volunteers working with Red Cross and Red Crescent agencies. More than 10,000 civilians had died as a result of civil war.