The marines are trying to help Haitian police disarm the rebels, but some rebels are resisting, suspicious that their opponents remain armed.
The capital Port-au-Prince is much calmer than the near-anarchy of a week ago, though fear of looting remains.
The UN, meanwhile, says Haiti faces a humanitarian crisis unless aid routes are quickly established and food sent.
On the political front, a seven-member council that will pick a new prime minister – the beginning of a new Haitian government – was chosen on Friday.
A spokesman for the Organization of American States said he hoped a replacement for Prime Minister Yvon Neptune – who has strong ties to the deposed president – would be chosen within days.
Information from outside Port-au-Prince is patchy, but US marines appear to have begun moving into rebel strongholds in the north, correspondents say.
Special teams from the Miami-based US Southern Command arrived in Cap-Haitien, on the northern coast, and Gonaives, on the western coast, a spokesman for the US forces was quoted as saying.
US marines say they are not trying to impose stability but are “supporting” Haitian police.
But some rebels are reportedly reluctant to hand over their weapons until their enemies – supporters of the exiled former President Jean-Bertrand Aristide – do the same.
In New York, UN Under Secretary General for Humanitarian Affairs Jan Egeland said most people were without health care because the hospitals were looted and the UN’s food distributions had come to a halt
He said the UN and other aid agencies were trying to negotiate with all parties and that the multinational interim force had agreed to help set up humanitarian corridors to allow aid to be distributed throughout the country.
In Port-au-Prince, the dust is slowly settling after the unrestrained killings, arson and looting of a week ago, the UN children’s fund (Unicef) told BBC News Online.
Eric Larouche said food and medical supplies were arriving and distribution had begun but he warned looting was continuing, and many people who needed hospital treatment were still too scared to leave home.
“If you go to a hospital, the doctors are very afraid,” he said.
“If you are delivering drugs and surgical equipment, they tell you it will only be looted overnight.”
Hospitals were one of the main targets for looters during the unrest.
On Friday, Canada announced it would contribute 450 troops to the interim international force currently being formed in Haiti. The force will help keep order ahead of a UN peacekeeping force expected to arrive in two or three months.
Defence Minister David Pratt said the troops would be accompanied by six helicopters.
He said he expected them to begin arriving in Haiti in about five days.
The troops will join 1,250 US marines, and more than 800 French, Canadian and Chilean troops already in place.
The US agreed at a meeting on Friday to lead the interim international force, the head of the French forces in Haiti, Gen Henri Clement-Bollet, told the AFP news agency.