The US has urged allies of exiled Haitian president Jean-Bertrand Aristide to rein in militants after two weeks of unrest and nearly 50 deaths.
A number of police officers have been abducted and beheaded
A US state department spokesman accused Mr Aristide's supporters of seeking to undermine the interim government formed after his overthrow in February.
Trouble began at the end of September at a rally by Aristide opponents in the capital, Port-au-Prince.
Aristide loyalists have blamed police for provoking the violence.
At least 46 people are known to have died, including five police officers. Most of them were killed in shootings in slums of Port-au-Prince where street gangs loyal to Mr Aristide are still strong.
Hospital records obtained by The Associated Press show that 17 people with gunshot wounds died on Monday alone.
"There was shooting everywhere," said witness Lovely Pierre-Louis, aged 19.
"I saw a man walking across that street with a boy, then the shooting came again, and he was on the ground with his head bleeding, and the boy was running."
Brazilian-led UN peacekeepers, already stretched by the need to keep order in the hurricane-hit city of Gonaives, have suffered two casualties - two soldiers were shot and wounded by Aristide supporters in the northern city at the weekend.
"Pro-Aristide thugs have murdered policemen, looted businesses and public installations, and terrorised civilians," said US state department spokesman Richard Boucher.
A "systematic campaign" was under way, he said, to "destabilise the interim government and disrupt the efforts of the international community to assist the Haitian people".
Support for Aristide is still strong in the slums
Mr Boucher appealed to leaders of Mr Aristide's Lavalas movement to "break with [its] legacy of violence and criminality".
He said the interim government led by President Boniface Alexandre
and Prime Minister Gerard Latortue represented "the best hope for progress" in Haiti.
Aristide supporters have accused the police of shooting dead two people at a rally on 30 September. Those deaths were followed by the killing of three police officers, whose headless bodies were found later that day.
Mr Aristide, in exile in South Africa, insists he is still the country's legitimate, elected leader and accuses the US of having helped to oust him.