Hundreds of United Nations troops have moved into Haiti's largest slum, Cite Soleil, sparking a major gun battle with suspected gang members.
Up to 700 blue-helmeted UN peacekeepers entered Cite Soleil, a coastal area of the Haitian capital Port-au-Prince.
UN officials said the force was aiming to secure a school being used by gang members as a base and weapons store.
UN troops have made regular incursions into Cite Soleil in the past, as they attempt to reduce violence in Haiti.
The Brazilian-led force of 8,000 has been deployed in Haiti since the ousting of former President Jean-Bertrand Aristide in 2004.
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The UN's commanding officer in Haiti, Maj Gen Carlos Alberto Dos Santos Cruz, said the troops were essentially mounting a police operation.
"We had a raid to try to arrest the criminals and recover their weapons they have inside this place," he told the Associated Press.
Much of the fighting was reported to be taking place in an area of Cite Soleil known as Boston.
That area is thought to be under the control of a gang leader known only as Evans.
Local radio reported heavy and sustained gunfire coming from the area of Cite Soleil, making an accurate assessment of casualty figures difficult.
At least four people were killed and six wounded during the UN's previous incursion into Cite Soleil, in late January.
There was further violence in December, with at least five people killed in similar clashes.
