Human rights groups say the police carry out routine executions of dissidents and weapons are often illegally funnelled to armed militia.
Robert Muggah of the Swiss-based Small Arms Survey, a non-profit group, said that last year the US effected the sale of thousands of weapons to the interim government headed by Gerard Latortue, despite a 13-year arms embargo. "They are meant to brace up a shaky security force, but the reality is they could actually undermine security by jeopardising an innovative disarmament effort just getting under way," said Mr Muggah, who has spent several months in Haiti interviewing diplomats and UN officials for a report.
The embargo was established after a coup that ousted the elected president Jean-Betrand Aristide, who was forced into exile for a second time last year. Washington, which had long under- mined his presidency, refused to help him. The weapons embargo remains in place.
Mr Latortue, installed following negotiations involving the US, France and Canada, complained the ban prevented him equipping the police.
But according to Mr Muggah, despite Mr Latortue's public protestations, a number of arms sales have gone ahead. His report says 5,435 "military-style weapons", including M-14 and other semi-automatic guns and 4,433 handguns worth $6.95m, were provided from the US.
Haiti is already awash with guns and violence. Human rights groups say supporters of Mr Aristide's political party, Lavalas, have been subjected to violence and oppression. The Haitian National Police (HNP) is also accused of carrying out a campaign of violence in the slums of the capital, Port-au-Prince.
Brian Concannon, a lawyer with the Institute for Justice and Democracy in Haiti (IJDH), which has links to the former government, said: "It is well-documented that the Haitian police routinely execute political dissidents and suspected criminals, so with them the guns are already in the wrong hands.
"But many officers - especially those illegally integrated from the rebel army - funnel arms to paramilitary groups that are even more brutal."
A recent report by the human rights programme at Harvard Law School said HNP members were "perceived variously as crooked, politicised, ineffective and violators of human rights".
It added that the UN stabilisation mission in Haiti (MINUSTAH) was not only complicit in HNP abuses, but that there were "credible allegations of human rights abuses perpetrated by MINUSTAH itself".
Media reports have identified several Haitian Americans allegedly involved in an arms sale, raising suggestions that the deal was privately organised and sanctioned by the US, rather than an official sale of weapons by Washington.
One of these, Joel Deeb, a self-styled "freedom fighter", told The Independent on Sunday he had been approached by Mr Latortue's nephew, Youri, and given $500,000 to buy arms.
"I was given half a million dollars in the form of a letter of credit," he said. "But there is an embargo. There has not been any deal yet. The money is frozen. Everybody is saying I have done something with the money, but it is still there."
A State Department spokesman said restrictions on arms sales remained. He said the US would not sell arms to Haiti but would consider lifting the embargo on a case-by-case basis to allow third parties to make the sales "to be helpful" to the interim government.