The situation in the Central African Republic's (CAR) capital of Bangui is "apocalyptic" as rebels close in the city.
Martin Ziguélé said there was daily fighting across the country and that he couldn't leave Bangui without an armed escort.
The UN says more than 200,000 people have fled their homes since the conflict erupted last month.
Rebel forces now control two-thirds of the country.
The CAR is one of Africa's poorest and most unstable countries, even though it is rich in resources such as diamonds and uranium. The UN estimates that about half the population is dependent on humanitarian aid.
The city is being defended by government forces backed by UN, Russian and Rwandan troops. A state of emergency was declared earlier this month.
Speaking in Geneva, a spokesman for the UN refugee agency, UNHCR, told reporters that rebel attacks had hampered humanitarian access to Bangui and that many people were now facing "dire conditions".
Disease was growing and some of those displaced were so desperate they were exchanging sex for food, spokesman Boris Cheshirkov added.
Meanwhile, a regional body of 12 member states has called for a ceasefire and urged armed groups to "disengage from the encirclement of Bangui".
The International Conference on the Great Lakes Region has warned that the crisis poses a serious threat to neighbouring countries.