GOMA, Democratic Republic of Congo (Reuters) - Militia fighters in eastern Democratic Republic of Congo attacked a U.N. peacekeeping base on Friday, triggering clashes that left two of the fighters dead and two peacekeepers slightly wounded, the U.N. mission said.

Thirty-four rebels from a Mai-Mai militia have been killed in fighting with Congo's army in the past week, local army spokesman Jules Ngongo said, a spike in violence he attributed to an army crackdown on the militia's harassment of local residents.

Friday's attack, in which two rebels were also wounded, was a rare frontal assault on U.N. forces charged with protecting civilians in Congo's east, where dozens of armed groups exploit mineral resources and prey on local residents.

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"Very early this morning, about 30 Mai-Mai attacked," mission spokeswoman Florence Marchal told Reuters, adding that U.N. forces drove off the assailants. It was not immediately clear which Mai-Mai group attacked nor what their objective was.

The Mai-Mai comprise a number of armed bands that originally formed to resist Rwandan invasions in the 1990s. They have since morphed into a wide variety of ethnic-based militia, smuggling networks and protection rackets.

Congo's mineral-rich eastern borderlands are a tinderbox of ethnic tensions and for more than two decades have been racked by violence that has often spilled across the country's borders.

President Joseph Kabila's refusal to step down at the end of his constitutional mandate last December has fueled further unrest in the country's east, where wars between 1996-2003 killed millions, and center, where an insurgency against the central government has killed thousands since last August.

Last week, U.N. forces in east Congo's South Kivu province intervened with helicopters and heavy machine guns to help beat back an advance by a separate rebel group on the strategic city of Uvira.

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The U.N. mission in Congo, known as MONUSCO, is the world's largest with some 18,000 uniformed personnel and a more than $1 billion annual budget.

(Reporting By Fiston Mahamba and Aaron Ross, editing by Larry King)