The UN mission in Haiti says four people were killed when its peacekeepers expanded their presence in the Cite Soleil slum of Port-au-Prince.
UN peacekeepers have an important presence in Haiti's capital
A spokesman said six other people were injured in what he described as exchanges of fire with criminals.
On Wednesday, when the operation took place, another UN spokesman had said there had been no reports of casualties or much of a reaction to the operation.
The peacekeepers are now holding a strategic location in Cite Soleil.
The stronghold is to be a centre for operations against armed gangs, a UN spokesperson told the BBC.
A local aid worker for the charity Medecins Sans Frontieres, Fabio Pompetti, told AFP news agency that three women had been wounded in Wednesday's clash and that one of them later died from her wounds.
UN peacekeepers, in Haiti since 2004, were sent to the country to maintain order after a revolt ousted former President Jean-Bertrand Aristide.
Hundreds of people have died in Port-au-Prince in clashes between rogue police officers, ex-rebels and gangs.
The Brazilian-led UN force includes more than 8,000 soldiers and police supported by some 1,000 civilian personnel.