UN peacekeepers are being redeployed to Goma in the Democratic Republic of Congo to protect the city from mutineers, the country's UN envoy says.
Roger Meece said he was determined to protect population centres in the east from advancing rebels of the M23 group.
Meanwhile, a source has told the BBC the army has retaken the two towns recently captured by the fighters.
The M23 mutineers say they have no intention of taking Goma and want to negotiate with the government.
The Congolese government and the UN say Rwanda is backing the rebels, a claim Kigali vehemently denies.
'New uniforms'The BBC has been told that Congolese soldiers have been seen entering the strategic town of Rutshuru, 70km (43 miles) north of Goma, captured by rebels over the weekend.
“Start Quote
End QuoteAs with most mutinies, the turmoil now spreading across the lush green hills of eastern Democratic Republic of Congo is, despite appearances, a calculated and calibrated affair”
Kiwanja, 20km further north, has also reportedly been retaken by the army after the M23 fighter left the town.
The rebels - who took up arms in April - named themselves the M23 after a failed peace agreement signed on 23 March three years ago.
On Tuesday, a rebel commander told the BBC they intended to hide out in the mountains until they were able to renegotiate a new peace deal and did not intend to take further ground.
They are supporters of renegade General Bosco Ntaganda, who is wanted for war crimes by the International Criminal Court (ICC).
Gen Ntaganda is an ethnic Tutsi - like the majority of Rwanda's leadership - and a recent UN report accused Rwanda of backing the rebels.
In a briefing to reporters in DR Congo's capital, Kinshasa, Mr Meece said it had been noted that the mutineers were wearing a new uniform, which indicated that they had the backing of a "foreign country".
Troublesome neighbours
- April-June 1994: Genocide of Tutsis in Rwanda
- June 1994: Paul Kagame's Tutsi rebels take power in Rwanda, Hutu fighters flee into Zaire (DR Congo)
- Rwanda's army enters eastern Zaire to pursue Hutu fighters
- 1997: Laurent Kabila's AFDL, backed by Rwanda, takes power in Kinshasa
- 1998: Rwanda accuses Kabila of not acting against Hutu rebels and tries to topple him, sparking five years of conflict
- 2003: War officially ends but Hutu and Tutsi militias continue to clash in eastern DR Congo
- 2008: Tutsi-led CNDP rebels march on North Kivu capital, Goma - 250,000 people flee
- 2009: Rwanda and DR Congo agree peace deal and CNDP integrated into Congolese army
- 2012: Mutiny led by former CNDP leader Bosco "Terminator" Ntaganda
He added that some M23 fighters who had defected had said that some of the rebels spoke English, indicating they were not from DR Congo.
"We are certainly absolutely determined to do everything possible to protect population centres against threats by the M23," Mr Meece said.
"We have since the beginning and we're continuing to make adjustments in deployment including bringing personnel that are available from elsewhere... and that certainly includes some from Kinshasa," the UN special representative to DR Congo said.
The M23 rebels defected from the army amid pressure on the government to arrest Gen Ntaganda.
An estimated 200,000 people have fled their homes since April, with about 20,000 crossing the border to Uganda and Rwanda.
Eastern DR Congo has been plagued by years of fighting.
In 1994, more than a million Rwandan ethnic Hutus crossed the border following the genocide in which some 800,000 people - mostly Tutsis - were slaughtered.
Rwanda has twice invaded its much-larger neighbour, saying it was trying to take action against Hutu rebels based in DR Congo. Uganda also sent troops into DR Congo during the 1997-2003 conflict.
