Rebels said they will be in the Haitian capital within days and Washington has sent Marines to protect its embassy after the country's second-largest city fell in a bloody armed revolt.
Rebel leaders said after seizing control of the northern city of Cap Haitien on Sunday they were ready to take the entire country and liberate it from President Jean-Bertrand Aristide's "slavery".
About 50 US Marines flew in on two C-130 Hercules transport planes to Port-au-Prince airport on a mission to protect the US Embassy and other US facilities in Haiti.
France, which ruled Haiti until 1804, joined several other foreign governments in telling its citizens to leave. The international airport was packed with people, including US missionaries, clamouring for flights out in stifling heat.
Mr Aristide's government said it was sending reinforcements north and repeated a plea for international help for its hopelessly outgunned police. About 60 people have died in the revolt that erupted on February 5th in the poorest country in the Americas.
Seizing their biggest prize so far, a rebel force of 200 on Sunday overran Cap Haitien, a city of about 500,000, putting anti-Aristide forces in control of much of the north.
At least 10 people were killed during the attack, including several rebels, government spokesman Mr Mario Dupuy said.
The relative ease with which the rebels took Cap Haitien heightened fears in the capital, where Mr Aristide still has plenty of supporters. Some set up barricades in streets around the teeming city to deter the rebels.
Prime Minister Mr Yvon Neptune repeated a government plea to foreign nations to send help for the Haitian National Police, a poorly trained group of perhaps 4,000 officers created in the mid-1990s when Mr Aristide disbanded the feared army.
Mr Aristide championed Haitian democracy in the 1980s and became its first freely elected leader in 1991, but is now accused of corruption and political thuggery by his opponents. He has vowed to stay on until his second term ends in 2006.
Tensions have simmered in the country since flawed parliamentary elections in May 2000.