Congolese rebel forces advanced on the eastern city of Goma on Wednesday, threatening to overwhelm government troops and a 17,000-strong UN force deployed to halt a return to all-out war.
Most civilian staff evacuated the UN headquarters north of the lakeside town for a compound on the shore of Lake Kivu nearer the Rwandan border and accessible by boat, staff said.
Four days of fighting since Tutsi rebels launched a new offensive on Sunday have displaced tens of thousands of civilians in North Kivu province, racked by continuous violence despite the end of Congo's 1998-2003 regional war.
Neighbouring Rwanda, whose 1994 genocide is intricately tied up in years of ethnic bloodshed in eastern Congo, accused Congolese forces of shooting across the nearby border.
"There was an incursion at the border when Congolese forces of FARDC (army) fired into Rwanda," Rwandan Foreign Minister Rosemary Museminali told Reuters in Rwanda's capital Kigali.
Rwandan troops did not pursue the Congolese troops, she said.
However, FARDC Colonel Jonas Padiri told a Reuters reporter at Kibati, at the entrance to Congo's Virunga National Park around 10 km (6 miles) north of Goma, that his forces had been targeted by incoming fire from the Rwandan side of the border.
"It started during the night and continued until just now. I have no orders to fire," Padiri said, as blasts and flashes of weaponry could be heard and seen coming from the border area.
"Well, you can see for yourself that it's coming from the Rwandan side," Padiri said.
The Reuters reporter could not verify which side of the border the blasts were coming from.
The U.N. peacekeeping force MONUC has been backing Congolese troops in operations against rebel Tutsi General Laurent Nkunda's National Congress for the Defence of the People (CNDP).
The CNDP accuses Congo's army of collaborating with the Democratic Forces for the Liberation of Rwanda (FDLR), which includes Hutu militias and former Rwandan soldiers responsible for Rwanda's 1994 genocide of Tutsis and moderate Hutus.
Retreat towards Goma
Congolese army troops pulled back to Kibati early on Wednesday after being attacked from the north and east, a day after rebels entered Rutshuru, 70 km (45 miles) to the north.
"Nkunda keeps saying that he is going to take Goma. All that stands between him and Goma are the Indians, and some of the Indian commanders are not interested in fighting," said a Western diplomat, referring to MONUC's Indian contingent.
The head of MONUC said it was "stretched to the limit".
"I obviously hope we can get some additional support as quickly as possible, so that we can move this process back on the right track," MONUC chief Alan Doss said late on Tuesday.
The UN Security Council expressed "grave concern" and called for an immediate ceasefire by all parties.
MONUC confirmed that CNDP fighters were present in Rutshuru on Tuesday but a spokesman for the mission said peacekeepers had not withdrawn from the town.
MONUC has pledged to defend Rutshuru and Goma, as well as the towns of Sake and Masisi, from attack.
But government soldiers retreating in chaos from Rutshuru turned on the peacekeepers on Tuesday night, attacking a MONUC base 35 km northwest of the town, wounding at least two peacekeepers, MONUC sources told Reuters.
Around 250,000 civilians have fled their homes in North Kivu since a January peace deal collapsed in August. Nearly 850,000 had been displaced in the previous two years, the United Nations says. Congo's 1998-2003 war and the resulting humanitarian crisis have killed an estimated 5.4 million people.