There were only 10 UN soldiers for every 10,000 inhabitants in the east
The head of UN peacekeeping has asked the UN Security Council for more than 3,000 extra troops to protect civilians in the eastern DR Congo.
Alain Le Roy said current peacekeeper numbers were not enough to protect civilians from violence perpetrated by rebel groups and the Congolese army.
There are 9,000 UN peacekeepers in the region, out of 17,000 nationwide.
The latest crisis began in August when rebels advanced towards Goma, which is now ringed with refugee camps.
UN Secretary-General Ban Ki-moon has called for a ceasefire to allow aid workers to reach 100,000 refugees cut off in rebel-held areas north of Goma.
The UN's peacekeeping mission in DR Congo, called Monuc, is its largest in the world, but Mr Le Roy said there were only 10 UN soldiers for every 10,000 inhabitants in eastern DR Congo.
FORCES AROUND GOMA
CNDP: Gen Nkunda's Tutsi rebels - 6,000 fighters
FDLR: Rwandan Hutus - 6,000-7,000
Mai Mai: pro-government militia - 3,500
Monuc: UN peacekeepers - 5,500 in North Kivu, 3,500 in South Kivu (17,000 nationwide)
DRC army - 90,000 (nationwide)
Source: UN, military experts
"We consider it's not enough," he said.
Recent fighting between government and rebel troops has displaced 250,000 people in the strife-torn region around Goma, the capital of North Kivu province.
Earlier, the rebel Congolese leader, General Laurent Nkunda, said he had formed an alternative administration in the area of eastern Congo that he controls.
In what observers say is his latest direct challenge to the central government, 12 ministers will take responsibility for a range of functions including police and security.
But the BBC's Mark Doyle in eastern DR Congo says the move appears to be pure propaganda.
Our correspondent says it may annoy the government but is likely to be insignificant unless the rebels follow it up with further military action.
'Looting and raping'
Meanwhile, government troops have faced fresh accusations that they have been ransacking villages and raping civilians.
UN spokesman Lt-Col Jean Paul Dietrich said looting began around Kanyabayonga, 100km (60 miles) north of Goma, on Monday afternoon and continued through the night.
UN spokesman Lt-Col Jean Paul Dietrich: 'We are stretched to the last man'
He also said that a rare night-time gunbattle had erupted for an hour on Tuesday night between rebels and the army near Kibati, 10km (six miles) north of Goma.
The area is home to 75,000 people who have been repeatedly forced to flee fighting.
Meanwhile, neighbouring Angola has denied reports that it was mobilising troops to send to DR Congo.
Angolan Deputy Foreign Minister Georges Chicoty said Angola was following events very closely, in case they needed to intervene.
Correspondents say there have been a number of recent unconfirmed reports of Angolan troops arriving in DR Congo to assist Congolese troops.
The presence of Angolans in the volatile region could be seen as a provocation by neighbouring Rwanda, raising fears that the fighting could spread.
Rebel leader Gen Nkunda claims to be fighting to protect his Tutsi community from attacks by Rwandan Hutu rebels, who fled to DR Congo after Rwanda's 1994 genocide.
The Congolese government has often promised to stop the Hutu forces from using its territory, but has not done so.
On Monday, Gen Nkunda threatened to take over the whole country if President Joseph Kabila's government continued in its refusal to negotiate with him. But a Congolese official said the government was still unwilling to talk to the rebel leader and accused him of war crimes.
The UN has accused both sides of war crimes during the latest violence.
Mr Kabila was elected president in 2006 in polls that were backed by the UN, and which international observers generally declared to be fair.