Fierce clashes have broken out between UN troops and gunmen in the Haitian capital, Port-au-Prince, reports say.
The UN has had a presence in Haiti since 1993
According to witnesses, between two and six people were killed in the clashes in the north of the city. UN officials were unable to confirm the deaths.
Reports describe gangs going on the rampage and kidnappings taking place.
An election in February brought some stability to Haiti but correspondents say there are signs the security situation could be deteriorating again.
"Two ladies who work in a nearby factory were killed by bullets," a young man who identified himself as Maxime told Reuters news agency.
"A man who was running away from the scene of the incidents was shot and fell on the ground," he said.
"It was a real war," another witness told Reuters.
UN spokesman David Wimhurst said peacekeepers had come under attack and returned fire.
A UN peacekeeping force has been in Haiti since 2004 to help restore order after former president Jean-Bertrand Aristide was forced to flee an armed coup.
The unrest follows a gang massacre in the Martissant neighbourhood of Port-au-Prince earlier this month, in which about 20 people died.