Congo's M23 rebels clashed with government troops for the first time in nearly two months on Friday, and neighbouring Rwanda said Congolese army shells had landed on its territory, raising tensions in the volatile region.

The fighting came after peace talks broke down in the Ugandan capital Kampala on Monday. It sent some 5,000 civilians fleeing across the border into Rwanda, a U.N. peacekeeping spokesman said.

General Sultani Makenga, M23's military commander, said the rebels came under attack at 4 a.m. local time (0200 GMT) at Kanyamohoro, around 15 km (10 miles) north of Goma, the largest city in eastern Democratic Republic of Congo.

"We are going to defend our positions," Makenga told Reuters by telephone.

Congolese army General Lucien Bahuma said M23 fighters had attacked first and fierce combat was continuing. The Congolese army was seeking to overrun rebel positions in Kibumba, just south of Kanyamohoro, officers said.

A Reuters reporter in Goma said fighting was intense and continuous on Friday as Congolese army troops bombarded rebel positions with heavy artillery and machinegun fire.

Rwandan army spokesman Brigadier General Joseph Nzabamwita said Congo's army fired three shells and small arms into Rubavu district, just over the border near the Rwandan city of Gisenyi. Border security had been tightened.

"There are many Congolese crossing into Rwanda," Nzabamwita said, adding that a 58-year-old Congolese woman had been evacuated after being struck by a bullet.

Rwanda's U.N. Ambassador warned the 15-nation council on Friday during a closed-door meeting that Rwanda would not tolerate shelling of its territory and was in a position to respond militarily, a U.N. Security Council diplomat said.

In late August, Rwanda accused Congolese troops of shelling its territory and the tensions between the two neighbours raised fears of a regional conflict.

South African and Tanzanian troops, part of a new 3,000-strong U.N. Intervention Brigade with a tough mandate to crush armed groups in eastern Congo, were present near the frontline but did not join the combat.

The United Nations mission in Congo (MONUSCO) asked a committee of military experts from the International Conference of the Great Lakes region - an 11-nation regional bloc - to investigate the reports of a shell landing in Kageyo in Rwanda.

"MONUSCO is on high alert and is closely monitoring the situation," Martin Nesirky, spokesperson for the U.N. secretary general, said in New York.

TALKS FAILURE

After the breakdown of the Kampala talks this week, the Congolese government said it had rejected the demands of the rebel movement for an amnesty for its leaders and its reintegration into the army.

M23 says it has no desire to rejoin the Congolese forces but is seeking an amnesty for its leadership.

In an address to congress on Wednesday, Congolese President Joseph Kabila threatened military force.

"The government will not continue to expose the lives of our compatriots to blind bombings and abuses of all kinds," he said. "(M23) is caught between the force of the army and a narrow political escape route."

It is the second time in three months that peace talks have faltered and combat has broken out.

In late August, government troops with the support of the U.N. Intervention Brigade forced M23 from positions just north of Goma.

M23 began as a mutiny by Congolese soldiers in early 2012. The rebels demand that the government fully implement the terms of a 2009 peace deal signed after a previous rebellion which was backed by Rwanda.

U.N. investigators have accused Rwanda of supporting M23, a charge that Kigali strenuously denies.