Government supporters in Haiti have set up roadblocks at key points in the capital, Port-au-Prince, and erected barricades at the presidential palace.
Rebels reportedly faced little resistance in Haiti's second city
Police are said to be abandoning their posts as rebels seeking to overthrow President Jean-Bertrand Aristide said they would move on the city.
The rebels have already overrun the country's second city, Cap-Haitien.
Amid the escalating violence, the United States has sent 50 marines to Haiti to protect its embassy.
The International Committee of the Red Cross says it is seeking to keep lines of communication open with all sides in the crisis.
The ICRC is also trying to avert a collapse of medical care in the country as the fighting spreads.
"We are speaking of one of the poorest countries in the world...hence any outbreak of violence creates havoc," said Yves Giovannoni, head of ICRC operations in Latin America and the Caribbean.
US Secretary of State Colin Powell has extended by 24 hours (until Tuesday 2200 GMT) a deadline for the political opposition in Port-au-Prince to agree to a power-sharing plan accepted by Mr Aristide, an opposition spokesman said.
The opposition is said to have rejected the plan and instead put forward its own proposal seeking Mr Aristide's departure.
The US marines are members of a quick-reaction unit designed to respond to security threats worldwide.
Senior US defence officials said their mission would be limited to providing protection for the US embassy and its officials.
The team's deployment follows the arrival in Haiti late last week of a separate small military assessment team, which was sent by the Pentagon to gauge security at the US embassy.
Reports on local media said an armed group has attacked a police station on the outskirts of the capital.
US marines will confine their actions to protecting the embassy
The BBC's Stephen Gibbs in Haiti says many in Port-au-Prince believe that an assault is imminent. Armed gangs loyal to Mr Aristide are roaming the streets, looking for rebels.
But he adds that the rebels would face a tougher battle in the capital than in the north, where they have swept out government forces.
President Aristide, who agreed to the international initiative which would see his powers reduced, has not been seen on Monday. He is believed to be in the barricaded presidential compound in Port-au-Prince.
France and Germany have advised non-essential nationals to leave the country.
"What we are seeing is a massacre developing," Foreign Minister Dominique de Villepin was reported as saying.
'Bad for Haitians'
The rebels, who were not invited to the international talks and have no links to the political opposition, say they will continue their action until all of Haiti is "liberated".
One of the rebel leaders, Guy Philippe, told foreign reporters in Cap-Haitien that Mr Aristide's days were numbered.
"I think that in less than 15 days we will control all of Haiti," he was quoted as saying by the Associated Press news agency.
All four of Cap-Haitien's police posts were ransacked and set on fire
Rebels said about 200 fighters swept into the port city of 500,000 people and captured its airport.
At least 11 people were reported to have been killed.
Haiti's neighbours have said they will not accept the overthrow of the democratically elected Mr Aristide, but his opponents accuse him of rigging elections four years ago and have demanded he stand down.