Two peacekeepers were among Sunday's dead, the worst day for the 10-month-old UN mission, officials said.
The Sri Lankan and Nepalese soldiers who died were the first peacekeepers killed in fighting since the UN troops arrived in June 2004 to stabilise the impoverished, volatile nation following the ousting of its leader.
The first clash erupted after UN troops raided a police station occupied by armed former soldiers in Petit-Goave, a stronghold for them about 72km west of Port-au-Prince, setting of a fierce gun battle, UN spokesman Toussaint Kongo-Doudou said.
"We lost one man," he said, adding that three other peacekeepers were injured and in stable condition. Two former soldiers died and 10 others were wounded.
Using a loudspeaker, the Brazilian commander of UN troops in Haiti, Lieutenant-General Augusto Heleno Ribeiro, tried for 20 minutes to get the former soldiers to surrender peacefully when they opened fire on UN troops, Kongo-Doudou said.
"We wanted to resolve this peacefully but our troops received a hostile response from the insurgents and so they responded with force," he said.
Gerard Nelson, a Petit-Goave resident, was sleeping about a block from the police station when he was awoken by gunfire and ran outside.
"There were bullets bouncing off the walls. People on the street were running to get out the way. It sounded like a war," Nelson said.
"There were bullets bouncing off the walls. People on the street were running to get out the way. It sounded like a war"
Afterwards, UN troops moved in on the building and removed at least one fallen peacekeeper on a stretcher, he said.
Later on Sunday, a group of Nepalese soldiers driving to the central town of Hinche exchanged gunfire with a different group of former soldiers, UN spokesman Damian Onses-Cardona said.
The disbanded troops killed one Nepalese and stole one of their vehicles. It was not clear if the former soldiers suffered any casualties.
Later on, Brazilian peacekeepers backed by Haiti's national police began advancing on nearby Terre Rouge, another stronghold of disbanded soldiers who occupy the town's police station. The troops reached about 3km outside the town before stopping at nightfall.
"They'll continue tomorrow," Onses-Cardona said. "It was an area not under control so basically it's a recover and control mission."
The clashes were the first major confrontation between the 7400-strong UN force and former members of Haiti's disbanded army, who helped oust former president Jean-Bertrand Aristide in a 1991 coup and again in an armed rebellion a year ago.
Large parts of Haiti have become
lawless and violent
Haiti, the Western Hemisphere's poorest nation, has been in turmoil for years. A US-led peacekeeping force was deployed after Aristide was forced into exile in February 2004, and that force was replaced by the UN peacekeepers in June.
Despite the UN presence, armed rebels and former soldiers still control much of Haiti's countryside and the peacekeepers have been criticised for failing to curb violence.
UN forces detained 35 former soldiers following Sunday's gun battle, Kongo-Doudou said.
"We are now in control of the police station," UN civilian police spokesman Jean-Francios Vezina said.
The soldiers, many well into their 50s with fading uniforms and ageing rifles, have continued to buck calls by the interim government and the UN force to disarm. Some former soldiers have demanded Haiti's army be reconstituted.
The ex-military has won in past confrontations. When Sri Lankan troops and Haitian police tried to force them from the former police headquarters that they took in December, a mob of supporters began throwing rocks at the peacekeepers, who retreated.
"If anybody tries to remove us from this base, we'll know what to do"
Michel Alophene, ex-soldiers' commander in Petit-Goave
In another stand-off that month, a different group took over Aristide's looted estate on the outskirts of Port-au-Prince.
They only withdrew from the property after the interim government agreed to give them back-pay for the 10 years they have been disbanded.
In a February interview with AP, the commander of the ex-soldiers in Petit-Goave hinted at a confrontation should UN troops attempt to intervene.
"If anybody tries to remove us from this base, we'll know what to do," former Sergeant Michel Alophene said.