The Democratic Republic of Congo’s government has rejected an ultimatum by rebels to start talks, as fighting continues near the city of Goma.
Rwanda, which denies arming the rebels, has accused Congolese forces of deliberately firing into its territory.
The M23 rebels have advanced to within a few kilometres of Goma, the main city in eastern DR Congo, causing tens of thousands of residents to flee.
The rebels have given government troops 24 hours to withdraw from Goma.
The UN Security Council has condemned the rebel advance – the most serious since July in the resource-rich region.
The UN is moving non-essential staff out of Goma as the rebels advance.
‘Fictitious forces’
The BBC’s Gabriel Gatehouse reports from Goma that the city is very tense.’
Soldiers have been taking up positions, while there has been firing at a roundabout in the city centre, he says.
One resident told the BBC they felt « abandoned » by UN peacekeepers.
« At least yesterday [Sunday], we could see UN helicopters bombing, fighting. But nothing today [Monday], » he said.
« We can hear shooting at the airport. We are fleeing not knowing where to go. »
Rwandan General Joseph Nzabamwita told the AFP news agency that DR Congo had used T55 tanks and mortar bombs to fire at the area around the airport in the border town of Gisenyi.
« Rwanda is exercising restraint as of now… We are concentrating on evacuating the affected population as we continue to assess the situation, » he said.
The Rwandan authorities said they had not responded to the fire, although local sources in Goma reported at least two explosions from ordnance apparently coming from the direction of Gisenyi, on the Rwandan side of the border, our correspondent says.
UN officials in Goma said they had heard similar reports.
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 ongoing battle for control of the region, which is rich in minerals.
Rwanda and Uganda strongly denied the allegation but both countries were heavily involved in the 1997-2003 Congo conflict.
The M23 rebels said in a statement that if the government failed to hold « direct political negotiations » within 24 hours, they would « pursue the resistance against the government of Kinshasa until it falls », AFP reports.
DR Congo government spokesman Lambert Mende said the rebels were « fictitious forces » backed by Rwanda to « hide its criminal activities » in the east.
« We prefer to negotiate with Rwanda, the real aggressor, » he said.
At the weekend, the UN Security Council demanded an end to outside support for the group, noting they were well-equipped.
‘Real threat’
The UK has urged British citizens to leave Goma and advised against all but essential travel to the whole country.
« Any British nationals in Goma should leave, and any in DRC should check the FCO’s [Foreign and Commonwealth Office] updated travel advice, » said Foreign Secretary William Hague.
Government forces and UN 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, Monusco, fought advancing rebels throughout Sunday, using rockets, cannon rounds and helicopter gunships.
« There is a real threat that the city could fall into the M23’s hands, » said spokesman Kieran Dwyer.
Last week, the UN and US imposed a travel ban and asset freeze on the group’s leader, Sultani Makenga
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