"The foreigners came shooting for hours without interruption and killed 10 people," said Johnny Claircidor, a resident of Bwa Nef.
"Then Belony's gang members started to exchange fire with them ... I personally counted 10 bodies."
In past gun battles in Haiti's crowded, maze-like slums, people have been struck by crossfire from both sides so it was not possible to immediately confirm who was responsible for the killings.
Sophie De la Combe, the spokesperson for the UN peacekeeping mission in Haiti, declined to provide a toll.
"No one was killed or injured on our side, but it's difficult for us to know for now how many bandits could have been killed or wounded," he said.
Jean Saint-Fleur, the director of Haiti's administrative police, said: "The operation was conducted to address the current insecurity caused by the recent wave of kidnappings in the capital Port-au-Prince."
He also said he was unable to give an official death toll from the Cite Soleil fighting.
"They came here to terrorise the population," said Rose Martel, a slum dweller, referring to the police and UN troops.
"I don't think they really killed the bandits, unless they consider all of us as bandits."