US marines said they were launching a joint operation with Haitian police to take weapons from armed groups.
Foreign troops arrived in Haiti more than a week ago after President Jean-Bertrand Aristide left the country.
A new prime minister, Gerard Latortue, has been appointed in a bid to fill Haiti’s political vacuum.
The move to disarm rebel groups comes as a United Nations mission of political, military, police and logistics experts is expected to arrive in Haiti.
The mission is intended to pave the way for a UN peacekeeping force to be deployed within three months.
But in a further sign of the continuing tensions in Haiti, US marines said on Wednesday that they had shot and killed at least two gunmen in an exchange of fire overnight.
A US military spokesman said the shoot-out happened when the marines came under hostile fire while patrolling near outgoing Prime Minister Yvon Neptune’s private home.
US Marines Colonel Charles Gurganus told reporters in the capital, Port-au-Prince, that his men and the Haitian national police would “disarm men who are illegally armed” in public.
“We will take as many weapons as we find on the street,” he added.
He called on Haitians to hand in weapons and pass on the names of those holding illegal arms.
The BBC’s Daniel Lak in Port-au-Prince says the operation could bring the marines into conflict with Haiti’s militias, especially supporters of ex-President Aristide.
The armed Aristide supporters now say they will revolt against any attempt to erase the legacy of their leader, who
is insisting that he was forced from power by the US and is still the rightful leader of Haiti.
Mr Aristide, now in the Central African Republic, is threatening legal action against the US and France, accusing them of abducting him and forcing him into exile.
On Tuesday, former Haitian Foreign Minister Gerald Latortue was appointed by a council of leading Haitians as the country’s new interim prime minister.
Mr Latortue – from the northern Haitian city and rebel stronghold of Gonaives – served in the government of Leslie Manigat, who was ousted in a coup in 1988.
An economist by profession, he has worked for the United Nations and had been living in Florida and working as a business consultant.
“I accept it [the nomination] with pleasure while recognising the complexity of the tasks that await me,” he was quoted as saying.
“I want to seize the occasion given to me to rally all citizens of the country on the basis of their competence, their honesty and their integrity to participate in the construction of a new Haiti.”
On Monday, Haiti swore in former supreme court chief Boniface Alexandre as interim president of the country.
Haiti is the Western hemisphere’s poorest country, with many people malnourished and unable to access health care, sanitation or drinking water.