United Nations forces and the Congolese army attacked rebel positions with helicopter gunships, armored personnel carriers

The fighting was some of the fiercest in the week since the newly created U.N. intervention brigade went on the offensive, and one Tanzanian peacekeeper was killed after the rebels aimed artillery fire at their position, the U.N. said in a statement. Seven other troops were also wounded, U.N. spokesman Farhan Haq said.

"I am outraged by today's killing of a United Nations peacekeeper from Tanzania by the M23," said Martin Kobler, the special representative of the secretary-general in Congo, who heads the peacekeeping mission. "He sacrificed his life to protect civilians in Goma."

The fighting is taking place nine miles (15 kilometers) from the provincial capital of Goma, a city home to nearly 1 million people that was briefly captured by the M23 rebels late last year.

The U.N. involvement in the latest flare-up of violence is in sharp contrast to November, when the U.N. peacekeeping mission, known as MONUSCO, stood by as the rebels overtook Goma because their mandate was only to protect civilians.

The stepped-up U.N. intervention brigade, created by the Security Council in March, is authorized to take the offensive against the rebels.

"It's already changing the equation. For now, I would shy away from calling it a game changer. It's certainly unprecedented not only for Congo, but for peacekeeping itself and the U.N. at large," said Timo Mueller, a Goma-based researcher with the Enough Project, an advocacy group active in eastern Congo.

Martin Nesirky, spokesman for U.N. Secretary-General Ban Ki-moon, said in a statement that Ban condemned the killing of the Tanzanian and violence against U.N. peacekeepers generally.

"The Secretary-General deplores in the strongest terms the killing and wounding of UN peacekeepers," the statement reads.

Even as forces pounded the rebels, U.N. officials continued to send mixed messages about the extent of their involvement, repeatedly saying they were merely "backing" or "supporting" the Congolese military, rather than leading the offensive themselves.

"The main engagement is by the (Congolese) forces," said Siphiwe Dlamini, a spokesman for the South African military, which contributed troops to the brigade. "We are retaliating and going on the offensive."

Lt. Col. Felix Basse, the military spokesman for the U.N. peacekeeping mission, also emphasized that U.N. forces were fighting alongside the Congolese army.

However, the president of the M23 rebel movement, Bertrand Bisimwa, who spoke by telephone, said the U.N.'s intervention brigade was on the frontline of Wednesday's fighting.

"It was the U.N. that was shooting directly at us, from their helicopters. It's the Tanzanian and South African (United Nations) troops that are on the frontline. It's them that we see first," he said.

As the U.N. mission takes its strongest steps yet to protect Congolese civilians, observers note the intervention brigade faces high expectations. It is already facing backlash from residents who say their heightened efforts still aren't enough to protect civilians from an onslaught of mortar fire.

Last weekend, scores of Goma residents took to the streets in anger after a barrage of mortar shells rained down on residential neighborhoods and killed several civilians. A U.N. car was set ablaze, and in the melee two protesters were killed.

"Given this outburst of frustration during these demonstrations, MONUSCO might feel pressured to take on M23 and be sucked into an active conflict, into active warfare," Mueller said. "There might be a momentum building up where MONUSCO has to prove its legitimacy and its effectiveness and has to show the population that it's actually doing something."

In a recent open letter, the charity Medecins Sans Frontieres, or Doctors Without Borders, also expressed concern about the mission's simultaneous mandate to "aid, protect and fight" in Congo. The U.N. vigorously defended its mandate, saying it could not "fold our arms and allow armed groups to kill the population."

In addition to seven U.N. troops wounded Wednesday, three others already have sustained injuries since the U.N. brigade began directly engaging rebels last week. One South African and two Tanzanians were hit by shrapnel Saturday, South Africa's military said in a statement Wednesday.

Angelo Izama, a Uganda-based analyst who runs a regional security think tank called Fanaka Kwawote, cautions though that failure to unseat the M23 from their current strongholds overlooking Goma would be a psychological blow to the Congolese military and its U.N. allies.

He said it was highly unlikely that the U.N. brigade would sustain the offensive if more and more of its troops were killed or wounded in combat.

"If they come under sustained attack, the U.N. will have no appetite for war," he said. "They will call time out."