GOMA, Democratic Republic of Congo (Reuters) - Shells fired by M23 rebels killed at least three people in Congo's eastern city of Goma on Saturday, the United Nations said, as Congo and Rwanda traded accusations over days of border clashes that have drawn in a new, robustly mandated U.N. force.
Two more civilians died during demonstrations that broke out in the wake of the shelling, the United Nations said.
Fighting between M23 and Congolese soldiers, after the rebels entered a security zone around Goma on Wednesday, has now killed at least seven civilians. On Thursday, a U.N. brigade formed to neutralize armed groups in Congo saw its first military action, firing artillery at the rebels.
Artillery fire has hit both sides of the border this week.
Rwanda twice invaded its much larger neighbor in the 1990s and sponsored Congolese rebels trying to topple the Kinshasa government. Millions have died since then in Congo's eastern borderlands, a patchwork of rebel and militia fiefdoms rich in tin as well as tungsten and coltan ores.
U.N. investigators have accused Rwanda of backing M23, an accusation Kigali has repeatedly rejected.
Congo's U.N. mission, MONUSCO, said two mortar bombs fired by M23 struck the Ndosho neighborhood of Goma - a city of 1 million people - on Saturday morning, killing three civilians and injuring several others.
A Reuters witness at the scene saw four bodies - a woman and three children.
"I have ordered the MONUSCO Force to react in the strongest terms possible to these horrifying and unqualifiable crimes," said Martin Kobler, head of the U.N. mission, which pledged last month to keep M23 out of range of Goma.
The rebels marched past peacekeepers to seize Goma briefly last year - an embarrassment for the United Nations that led to the creation of the 3,000-member Intervention Brigade, made up of South African, Tanzanian and Malawian troops.
M23 denied responsibility for Saturday's mortar attack.
"(The army) is doing this because it wants to draw MONUSCO into the combat on its side," M23 spokesman Amani Kabasha said.
U.N. INJURED, PROTESTERS DEAD
Peacekeepers, including members of the Intervention Brigade, were again involved in the fighting on Saturday, pounding M23 positions in Kibati, 11 km (7 miles) north of Goma, in response to the shelling of Goma.
"We have used our attack helicopters to destroy M23 positions around Kibati," MONUSCO military spokesman Lieutenant-Colonel Felix Basse said by telephone from Goma.
Two Tanzanian U.N. soldiers and one from South Africa were injured when M23 mortar shells struck MONUSCO positions in Munigi, around 4 km (2.5 miles) north of Goma, the mission said.
Thousands of residents took to the streets to protest against the attack on Goma, carrying the body of one of the victims towards the Rwandan border in protest.
Then, in a reflection of growing frustration with MONUSCO, they confronted peacekeepers shouting: "This time you have to leave!"
Police fired teargas to disperse them. Protesters responded by hurling stones.
A Reuters witness heard the sound of gunfire during the demonstrations but said it was unclear who was shooting. MONUSCO later confirmed that two civilians had been killed and called for an investigation by the United Nations and Congolese police.
Congolese Information Minister Lambert Mende on Saturday accused Rwanda of direct involvement in this week's fighting.
"Rwandan troops crossed the border (on Thursday) ... and were with M23. That is still the case as far as we know," he said.
Rwandan Defense Ministry spokesman Brigadier General Joseph Nzabamwita denied any army involvement, calling the accusation "outrageous, old, spurious and outlandish".
Kigali said five mortar bombs had fallen on Rwandan villages on Friday, following a rocket the previous day, and blamed Congo's army.
(Corrects death toll (7, not 6) in first bullet point)
(Additional reporting by Chrispin Mvano in Goma, Pete Jones in Kinshasa and Jenny Clover in Kigali; Editing by Joe Bavier and Kevin Liffey)