M23 rebels in eastern DR Congo have moved to within a few kilometers of the provincial capital, Goma.
Although the rebels say they have no plans to seize the city, residents fear the city could fall anytime.
The UN Security Council over the weekend condemned rebel attacks and demanded an end to outside support for the group, noting they were well equipped.
United Nations and government troops still control Goma’s airport, but the UN says the humanitarian situation is worsening, with some 60,000 internally displaced people fleeing the fighting.
The UN said more than 10,000 fleeing civilians were seen passing near the airport on Sunday.
The rebels captured the town of Kibumba 30km (19 miles) north of Goma, on Saturday, and have since edged closer towards the North Kivu provincial capital, which lies close to the borders with Rwanda and Uganda.
The UN said its peacekeeping forces, Monuscu, fought advancing rebels throughout Sunday, using rockets, cannon rounds and helicopter gunships.
Monuscu has 270 international staff in Goma, while there are an additional 355 international UN staff in the city.
Kigali denies Congolese allegations that the rebels are backed by Rwandan troops and heavy weaponry though UN experts say they have evidence of Rwandan support for the rebels, and this week asked the Security Council to sanction senior Rwandan officials as a result.
Nearly 500,000 people have fled their homes since April when the rebels mutinied from the army.
Uganda closed the Bunagana border crossing near Goma on Tuesday after a request from the DR Congo government, which said the M23 was illegally raising money from people travelling between the two countries.
Last month, a UN panel of experts said Rwanda and Uganda were supplying M23, also known as the Congolese Revolutionary Army, with weapons in what is seen as an on-going battle for control of the region, which is rich in minerals.
Rwanda and Uganda strongly deny the allegation – Rwanda has called on both sides to stop fighting, saying stray bullets have fallen on its side of the border, injuring civilians.
The UN and US imposed a travel ban and asset freeze earlier this week on the group’s leader, Sultani Makenga.
The UN has a large force in DR Congo to help the government establish its authority in the mostly lawless east.