Battles rage near key Congo town
The army is finding it difficult to locate the rebel fighters
Government troops and UN forces have been involved in fierce battles against Tutsi rebels in eastern Democratic Republic of Congo.
Reports from the region suggest the strategic village of Kibumba, north of the regional capital Goma, is the latest to have fallen to the rebels.
Renegade General Laurent Nkunda's fighters say their next target is Goma, where thousands of civilians have fled.
The head of the UN mission in Congo says he desperately needs more troops.
The UN Security Council late on Tuesday called for an immediate ceasfire and issued a statement in which it "strongly condemned the offensive operations" against its peacekeepers.
UN forces have been using attack helicopters and tanks to try to stop the rebel advance, but they say the rebels work in small groups, making them difficult to locate and repulse.
Alan Doss, the head of the UN mission in DR Congo (Monuc), told the BBC his forces were stretched to the limit and needed urgent reinforcements.
He said his troops would do their utmost to stop major towns in the region from falling to the Tutsi rebels under Gen Nkunda.
We simply cannot send teams out into the countryside... it's too dangerous; it's anarchy
UNHCR's Ron Redmond
"We are going to remain there, and we are going to act against any effort to take over a city or major population centre by force," he said from Kinshasa, the Congolese capital.
Monuc has 17,000 troops in DR Congo - the largest peacekeeping force in the world - but has come under criticism from residents in the east of the country for being unable to protect them.
The head of UN peacekeeping, Alain Le Roy, briefed the Security Council, and said the appeal for more troops had been "heard clearly by all member states".
The towns of Rutshuru and Rubare have also been threatened by the rebel forces.
Rutshuru houses tens of thousands of displaced people and dozens of aid workers are usually based there.
Some of them told Associated Press news agency that they could hear artillery fire near the town but that crowds of anxious civilians and Congolese soldiers were blocking UN efforts to evacuate them.
A UN worker said thousands of people were fleeing Rutshuru heading toward the Ugandan border to the north.
Thousands flee Congo fighting
An estimated 20,000 people have already fled towards Goma, many of them having left a refugee camp in Kibumba as the fighting approached on Monday.
About 200,000 people fled their homes after fighting resumed in the area in late August.
The United Nations says many refugees are malnourished and some are dying of hunger.
Ron Redmond of the UN refugee agency says it almost impossible to reach those in need of help.
"We simply cannot send teams out into the countryside. There's too much fighting going on; it's too dangerous; it's anarchy," he said.
"There are already some 800,000 to a million people internally displaced in this region, so it's really a huge population in need of help."
Rwanda has been accused of backing Gen Nkunda, who left the army and launched his own low-level rebellion after DR Congo's civil war ended.
He says he is fighting to protect the minority Tutsi community from the Hutu militia which carried out the 1994 genocide in Rwanda.
A peace deal was signed in Goma between the government and various rebel groups at the end of January.
Although he signed the deal, Gen Nkunda - whose main strongholds are in Kichanga in the Masisi Mountains and Bunagana town bordering Uganda - has always refused to disarm while Rwandan Hutu rebels still operate in the area.
Many Congolese say that Rwanda is helping Gen Nkunda's forces, something that both Rwanda and the UN have denied.