UN troops in Haiti lost two peacekeepers in their biggest battle since starting the mission, as they shot it out with ex-soldiers.

The clash killed four people, including two peacekeepers – a Sri Lankan and a Nepali – who were the first UN troops killed since the mission began in June 2004.

UN spokesman Toussaint Kongo-Doudou said three other peacekeepers and 10 former members of the country's disbanded army were wounded as they battled in Petit-Goave, about 70 kilometres west of Port-au-Prince.

A group of former soldiers of the disbanded Haitian Army stand in formation at their base in Petit Goave. (AP Photo)

The gunfire started when UN troops entered Petit-Goave before dawn to try to oust the ex-soldiers from a police station.

"We wanted to resolve this peacefully but our troops received a hostile response from the insurgents and so they responded with force," said Lt.-Gen. Augusto Heleno Ribeiro, the Brazilian commander of UN troops in Haiti.

Ribeiro said he tried for 20 minutes to get the former soldiers to surrender peacefully when they opened fire on UN troops.

Then later on Sunday, a group of Nepalese soldiers driving to the central town of Hinche exchanged gunfire with another group of ex-soldiers.

It was the first major clash between the 7,400-member UN force and the ex-soldiers, who helped oust former president Jean-Bertrand Aristide in a 1991 coup and again in a widespread rebellion a year ago.

Aristide disbanded the army in 1995, after the United States restored him to power.

However, the ex-soldiers control much of Haiti's countryside and some towns, despite being urged to disarm by the interim government and the United Nations.