* Days of shelling kill at least six civilians
* Congo accuses Rwanda of sending troops to support rebels
* Kigali accuses Congo army of shelling Rwandan villages
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 U.N. peacekeeping 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 rebels.
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 the bodies of four victims, including 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.
M23 denied responsibility for the attack, which sparked demonstrations by Goma residents who carried the body of one of the victims to the Rwandan border as a protest.
"(The army) is doing this because it wants to draw MONUSCO into the combat on its side," M23 spokesman Amani Kabasha said.
Artillery fire has hit both sides of the border this week.
Congo's Information Minister Lambert Mende on Saturday accused the Rwandan army of participating directly 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 120mm mortar bombs fell on Bukumu, Kagezi, Kageyo and Rusura villages in Rwanda on Friday, a day after a 107mm rocket caused property damage in Bugu, and claimed the villages were deliberately targeted by the Congolese army.
"Acts of provocation that endanger the lives of Rwandan citizens will not remain unanswered indefinitely," the ministry said in a statement.
U.N. officials have said that M23 was behind the 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 U.N. Intervention Brigade made up of South African, Tanzanian and Malawian troops.