Twelve young men were found shot to death Tuesday near Morne-a-Bateau, a town 31 kilometers west of the capital, the private Haitian Press Agency reported.
The agency and local radio said the bodies apparently had been taken to the site and dumped.
Reports of the massacre could not immediately be confirmed, underlining the importance of human rights observers, UN officials said.
"It could very well just fall through the cracks," said UN spokesman Eric Falt, one of 92 observers scheduled to leave Wednesday on orders of the military-installed government.
Colin Granderson, the chief of the UN Organization of American States mission, said threats forcedobservers to go. They were leaving with feelings of indignation and sadness, he said.
"No one will be able to witness the extent of the repression in Haiti," he said. U.S. Embassy investigators would be dispatched to the scene, embassy spokesman Stanley Schrager said Wednesday.
President Bill Clinton once again raised the possibility of U.S. intervention to restore President Jean-Bertrand Aristide, who was overthrown in 1991.
Haitian Foreign Minister Charles David said the observer mission was illegal, and its expulsion shouldn't justify military intervention.
"Haiti is not a threat to anyone," he said. "It's black and white. They are not wanted here. So they are going to have to leave."
The UN Security Council condemned the expulsion order Tuesday as "provocative" and a "serious escalation" in the Haitian crisis.
"If anyone thinks they're going to outbluff the international community or the United States, this time they're not going to," Schrager said.
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