Neighboring Rwanda's Tutsi-led government has repeatedly denied Congo's accusations that it has sent troops across the border.
Le Roy said that his request for additional forces for the UN mission in DRC (Monuc) had "been heard clearly by all member states".
He said that Joseph Kabila, the DRC president, also made a request for the dispatch of a "multinational force" to beef up Monuc on Tuesday.
But the 15-member Council merely issued a non-binding statement in which members expressed "great concern at the resurgence of violence in eastern DRC and strongly condemned the offensive operations".
Le Roy stressed that Monuc urgently needed an special forces unit as well as air support.
"We have seen statements by the CNDP [Nkunda's National Congress for the Defence of the People] that they want to take Goma," he said.
"This is very worrying and would be very dangerous for the stability of the region."
Ban Ki Moon, the UN's secretary-general, said on Wednesday that he had dispatched two of his senior aides to talk to all parties to the conflict.
|One million Congolese could be displaced from North Kivu [AFP]
"First and foremost, the fighting must be stopped," Ban told reporters on the sidelines of a UN-sponsored conference on migration in Manila.
"And I am deeply concerned about the civilian casualties as well as increasing number of internally displaced persons," he said.
Joe Bavier, an independent journalist in Kinshasha, told Al Jazeera: "There are at least 850,000 internally displaced people from North Kivu province, and that was before latest wave of fighting started in August. We're talking of another 250,000 displaced since.
"If the UN is forced to withdraw from North Kivu, you're talking about nearly a million displaced Congolese, with basically no protection from what are about about a dozen armed groups in North Kivu."
Speaking to reporters by video link from Kinshasa, Alan Doss, the chief of Monuc, vowed to do his utmost to stop the anti-government forces from seizing major population centres in the area.
"We will do whatever is necessary to defend the city [Goma] and prevent a humanitarian disaster."
On Tuesday, Monuc used helicopter gunships in an attempt to halt the anti-government forces' advance.
Doss said that less than 6,000 troops from the 17,000-strong Monuc were deployed against Nkunda's irregular forces , which he said were breaking up into small units and attempting to sow panic and confusion among civilians.
|Monuc's chief said the UN would do what they can to prevent a humanitarian disaster [AFP]
He complained that some elements of the DRC army abandoned their positions "without warning us," and spoke of "shortcomings" in the DRC military's chain of command.
Jean-Maurice Ripert, France's UN ambassador, said: "For the moment, Monuc has to react with the means it has now.
"Any kind of reinforcements will take time," he said.
The North Kivu province has been the site of fighting between government and rebel forces since late August, after the breakdown of the ceasefire signed in January.
In the last few days Nkunda loyalists launched an offensive that put government forces on the run, and brought them to Kibumba, some 30km north of Goma.
The Kinshasa government has accused neighboring Rwanda of actively supporting Nkunda, a charge Kigali has denied.