Fighting resumed Friday between the army and rebels in northeastern Democratic Republic of Congo, both sides and the United Nations said, just days after the latest effort at peace talks fell apart.
The clashes, which forced thousands of civilians to flee, took place some 25 kilometres (16 miles) north of Goma, a strategic city of one million people in the mineral-rich but volatile east, rebel spokesman Vianney Kazarama told AFP.
Neighbouring Rwanda accused DR Congo troops of firing three shells over the border into its territory and threatened to retaliate if the firing continued.
Kazarama said the army attacked rebel positions early Friday, but the military insisted it came under attack first -- a claim supported by a source from the UN peacekeeping mission in the country, MONUSCO.
The violence continued through the day, according to a statement from MONUSCO, which went into action alongside Kinshasa's troops.
MONUSCO said it was "extremely concerned" about the resumption of hostilities, calling on the M23 rebels to return to the negotiating table.
"I am serious in our efforts to protect civilians and neutralise all armed groups to bring back peace and stability", said MONUSCO chief Martin Kobler.
A local human rights activist saw UN helicopters flying over Kibumba, an M23 stronghold under attack, and said civilians were fleeing towards the border with Rwanda.
The UN Security Council in March approved the deployment of a special African force of up to some 3,000 troops, under MONUSCO command and led by a Tanzanian general, with an unprecedented offensive mandate to target rebel groups in DR Congo.
The Rwandan army said that three shells had fallen on its territory, injuring a civilian.
"If they are not ready to stop this, we will immediately act and it will hurt," Rwanda's UN ambassador Eugene Richard Gasana told AFP after UN Security Council talks on the heightened tensions.
Rwanda's ministry of refugee affairs said between 2,500 and 3,000 people had fled into the country via two border posts.
'Sabotage of the peace process'
On Monday, the rival sides both announced a halt to peace talks taking place in the Ugandan capital Kampala despite UN pressure to end the year-and-a-half-old rebellion ravaging eastern DR Congo.
According to the Congolese government, the talks were suspended due to disagreement over the extent of an amnesty for the army mutineers and their reintegration into the national army.
Backed by the international community, DR Congo's government is refusing to give amnesty to about 80 leaders of the M23 rebels and to enlist these men into military ranks.
In a statement Friday, M23's political spokesman Amani Kabasha accused the Kinshasa government of carrying out a "wholesale sabotage of the peace process in Kampala".
Members of the M23 group are mainly Tutsi fighters from an earlier rebellion who were incorporated into the army in 2009 and then mutinied in 2012, claiming that Kinshasa had failed to keep its part of a peace deal.
The rebels took control of Goma, capital of North Kivu province, for more than a week late last year before withdrawing under international pressure.
Kinshasa has long accused Rwanda of pulling the strings behind the rebellion and UN experts have even said that the M23's "de facto chain of command" was topped by Rwanda's defence minister.
The human rights activist said that at Kabagana, close by the Rwandan border, he saw Rwandan troops "coming in... to reinforce the M23".
Kigali has vehemently denied accusations that it is arming, financing the rebels -- and even supporting them with its own forces.