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.  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. 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.  