The area in question, Cité Soleil, is Haiti's (and so possibly the world's) poorest and most violent slum, and a hotbed of Aristide support. It has been isolated from the rest of Haiti for most of this year. The report I heard stated that civilians had been killed in the crossfire during a gun battle between The UN and local gangs.
I have also heard reports of summary executions, carried out by the Haitian National Police (not the UN):
link (external - login to view)
Monday, April 4, 2005 Updated at 5:13 PM EST
Port-Au-Prince, Haiti — UN troops battled street gangs in firefights near a lawless Haitian slum, the latest in a series of clashes between peacekeepers and armed groups that some fear could disrupt fall elections.
At least five gang members wounded or killed in the gunfight late Sunday in the Port-au-Prince slum of Cité Soleil, soldiers said. But UN military spokesman Lieutenant Colonel Elouafi Boulbars could not immediately confirm the casualties. No UN peacekeepers were hurt, he said.
Peruvian peacekeepers were patrolling the western edge of Cité Soleil when armed men in four cars opened fire, Lt.-Col. Boulbars said. Troops returned fire and chased the cars through the slum, a hotbed of gang violence between supporters and opponents of ousted President Jean-Bertrand Aristide.
“We destroyed two of their vehicles and badly damaged the other two, but they were able to escape,” Lt.-Col. Boulbars said.
The clash was the fiercest since peacekeepers first surrounded Cite Soleil on Thursday as part of a promised crackdown on violence that has surged in the capital more than a year after an uprising ousted Aristide.