* Days of shelling kill at least six civilians this week
* Congo accuses Rwanda of sending troops to support rebels
* Kigali accuses Congo army of shelling Rwandan villages (Adds U.N. spokesman quotes, injured peacekeepers, protest details)
By Kenny Katombe
GOMA, Democratic Republic of Congo, Aug 24 (Reuters) - S hells 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.
Fighting between M23 and Congolese soldiers, after the rebels entered a security zone ringing Goma on Wednesday, has killed at least six civilians. On Thursday, a U.N. brigade formed to neutralise armed groups in Congo took 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 neighbour 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 tungstun 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 neighbourhood of Goma - a city of one 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, who heads the U.N. mission which pledged last month to keep M23 out of range of Goma.
Peacekeepers, including members of the U.N.'s 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.
Three U.N. soldiers were slightly wounded and were being treated at MONUSCO's hospital in Goma, Basse said, declining to give further details on their identities or nationalities.
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.
TEARGAS AND STONES
Following the mortar attack on Goma, thousands of residents took to the streets, carrying the body of one the victims towards the Rwandan border in protest.
Then in a reflection of growing frustration with MONUSCO, which many Goma residents believe is not doing enough to protect them, they marched towards U.N. installations.
"This time you have to leave," they shouted at U.N. peacekeepers and burned tyres in the streets before police fired teargas to disperse them. Shots were also fired in the air. Protesters responded by hurling stones.
Congo's 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. Fighting is ongoing and we have found the bodies of Rwandan soldiers," he said.
Rwanda's defence ministry spokesman denied any army involvement in the fighting. "Those are outrageous, old, spurious and outlandish accusations that do not justify senseless repeated provocative firing on Rwanda territory," Brigadier General Joseph Nzabamwita said.
Kigali said five mortar bombs fell on Rwandan villages on Friday, a day after a rocket damaged property in another village, and said the villages were targeted by Congo's army.
U.N. officials have said that M23 was behind Thursday's shelling of both Goma and Rwanda. On Friday, France condemned the rebels targeting of U.N. positions and civilian areas, saying doing so constituted war crimes.
M23 dealt a serious blow to the image of MONUSCO - at 17,000-strong, the world's largest U.N. mission - last November by marching past U.N. soldiers to briefly seize Goma, and that led to the establishing of a 3,000-member Intervention Brigade, made up of South African, Tanzanian and Malawian troops. (Additional reporting by Chrispin Mvano in Goma, Pete Jones in Kinshasa and Jenny Clover in Kigali; Editing by Joe Bavier and Louise Ireland)