The US-led multi-national force in Haiti says it will round up illegal weapons in an effort to restore order and end weeks of violence.
The US has deployed just over 1,600 troops to Haiti
US marines said they were launching a joint operation with Haitian police to take weapons from armed groups.
The move came as the United Nations children's agency, Unicef, called for urgent action to save Haiti's children from malnutrition and disease.
The agency said thousands were in need of medical attention.
The deputy director of the Unicef emergency programme, Eric Laroche, said many children had died of pneumonia or diarrhoea following the armed uprising against President Jean-Bertrand Aristide, who left the country at the end of last month.
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.
Aristide accused France and the US of forcing him out
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.
A new interim prime minister, Gerard Latortue, has been appointed in a bid to fill Haiti's political vacuum.
An economist and former foreign minister, he has worked for the United Nations and has been living in Florida and working as a business consultant.
He was expected to arrive back in Haiti late on Wednesday and begin the task of forming a new government.
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.
"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.
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.