M23 rebels declare ceasefire and say they have withdrawn from frontline but fears of conflict escalating remain

Rwanda has accused the Democratic Republic of the Congo (DRC) of firing bombs and rockets into its territory, warning that "provocation can no longer be tolerated" and raising the prospect of all-out war. Such an outcome could result in the UN's first offensive force – whose arrival has inadvertently triggered the crisis – being dragged into a regional conflict or forced into a humiliating retreat.

The UN intervention brigade has been backing the Congolese army against the M23 rebel movement – alleged to be receiving support from Rwanda – in some of the most intense fighting of the past year near Goma in eastern Congo. There was hope of a respite on Friday when the M23 declared a ceasefire and said it had pulled back three miles from the frontline.

The M23 president, Bertrand Bisimwa, told the Associated Press: "We have decided to decree a unilateral ceasefire and we have started pulling our forces out of Kanyaruchinya in order to allow the investigation into the shelling. This announcement, which was made unilaterally, is meant to allow the Congolese to return to the negotiating table … and to give peace a chance."

But there were signs that the ceasefire alone would not resolve underlying tensions between Congo and Rwanda. A Congolese government spokesman, Lambert Mende, said: "It's our opinion that the only interesting proposition would be to see M23 demobilised, and to see them dissolve and cease all military action. Any other proposal is unacceptable."

Rwanda alleges that the Congolese army, or FARDC, has launched 34 attacks on its territory in the past week. A bomb fired on Thursday morning killed a woman and seriously injured her two-month-old baby in a market in the town of Rubavu, it claimed.

Louise Mushikiwabo, Rwanda's foreign affairs minister, said: "The persistent shelling of Rwandan territory is unacceptable, as it would be to any sovereign nation. Rwandan civilians are being targeted by DRC forces. We have remained restrained for as long as we can but this provocation can no longer be tolerated. We have the capacity to determine who fired at us and will not hesitate to defend our territory. Rwanda has a responsibility to protect its population."

Kigali has repeatedly urged the Congolese government to stop the attacks, but they have increased in frequency and intensity, Mushikiwabo said. She also claimed that the Congolese army was collaborating with the Democratic Forces for the Liberation of Rwanda (FDLR), a group composed mainly of remnants of the Hutu militias that perpetrated the 1994 genocide.

"It is hypocritical for the international community to talk about protecting civilians when FARDC together with FDLR are causing harm to our citizens as if the lives of Rwandans have no value. The attacks by FARDC and FDLR have now reached another level."

Mushikiwabo later challenged a report by al-Jazeera with a tweet that said: "No, al-Jazeera: Rwandan troops are not in DRC (yet); when they are, you will know, and no more need for erroneous reports."

With uncertainty around how Rwanda might retaliate, the UN secretary general, Ban Ki-moon, has reportedly spoken with the president, Paul Kagame to urge restraint.

Both Congo and the UN claim that the attacks on Rwanda are being carried out by the M23. Edmond Mulet, the deputy UN peacekeeping chief, reportedly briefed the security council that the UN mission had seen only M23 rebels shelling Rwanda.

Some believe that the M23 could be targeting Rwanda to give it an excuse amid the fog of war for blaming Congo and mounting an invasion. Lambert Mende, a Congolese government spokesman, told the BBC that the rebels were firing on to Rwandan territory "in order to give Rwanda a pretext for coming in openly in this war".

Stephanie Wolters, an independent political analyst and Congo expert, said: "I don't have any evidence to that effect but I wouldn't rule it out at all.

"Rwanda is going to use it as an excuse to be as aggressive as it can be. It's very possible that in the coming days we'll see this turning into a war between two countries. I think Rwanda think they have nothing left to lose and they might go in. It may be a full-scale invasion unless something defuses it, but I don't know what that would be."

Such a scenario would leave the 3,000-strong UN brigade caught in the middle of two armies, faced with a dilemma to fight or flee. In fighting this week a Tanzanian soldier was killed and three South Africans injured.

Although Rwanda is one hundredth the size of its neighbour, its military is better organised and twice invaded during the 1990s, forcing the overthrow of the Congolese dictator Mobutu Sese Seko.

The current president, Joseph Kabila, is similarly vulnerable. Wolters predicted: "If Rwanda deploys in the east, it is likely to spell the beginning of the end of Kabila because the FARDC is incapable of defending its territory."

On Friday the M23 said they would stop fighting and withdraw from frontlines immediately to allow an investigation into the attacks on Rwanda. "We have just asked our forces to withdraw from the Kanyaruchinya front line and cease combat so as to allow for an investigation into who was shelling Rwanda," the M23 leader Bertrand Bisimwa told French RFI radio.

Bisimwa said the M23 would hold on to other positions and he asked Congo's government to resume negotiations to end the crisis.

Rwanda this week blocked a joint US-French proposal to impose UN sanctions on two senior M23 commanders, arguing that the evidence against the men was weak.