The United Nations likely refused to accept legal responsibility for a cholera outbreak in Haiti that has killed thousands in the past six years because it was a position pressed by the United States, an independent U.N. investigator said on Tuesday.

Haiti was free of cholera until 2010, when U.N. peacekeepers dumped infected sewage into a river. Since then, more than 9,000 people have died of the disease, which causes uncontrollable diarrhea, and more than 800,000 people have fallen ill.

Philip Alston, the independent U.N. Special Rapporteur on Extreme Poverty and Human Rights, told a U.N. committee on Tuesday that the world body has refused to publicly state the reason for its legal stance.

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"There is reason to believe that the position adopted by (the U.N. Office for Legal Affairs) in 2013 was consistent with views strongly pressed at the time by the United States," said Alston, noting that the United States has a strong interest in the issue as a close neighbor of Haiti and the largest contributor to the U.N. peacekeeping budget.

He said the United States seemed to believe that the United Nations "must follow American legal practice, which generally takes the view that legal responsibility should never be accepted when it can possibly be avoided because one never knows the consequences for subsequent litigation."

The U.S. mission to the United Nations was not immediately available to comment on Alston's remarks.

After a lawsuit was filed in the United States on behalf of cholera victims, the United Nations said in 2013 that claims for compensation were "not receivable pursuant to Section 29 of the Convention on the Privileges and Immunities." A U.S. federal appeals court upheld the United Nations' immunity in August.

Under Section 29, the United Nations is required to make provisions for "appropriate modes of settlement" of private law disputes to which the world body is a party or disputes involving a U.N. official who enjoys diplomatic immunity.

Alston said "if there is a private law issue, in other words negligence, then the U.N. is obligated to accept the claims that they make and to process them in some appropriate way - that's what the U.N. is refusing to do."

He said the United Nations could abide by its obligations under the treaty without jeopardizing its immunity.

U.N. Secretary-General Ban Ki-moon has said the world body has a "moral responsibility" to help the cholera victims with material support. He is working on a new approach to dealing with cholera in the Caribbean country, and the United Nations hopes to raise enough money to pay some $10,000 to each of the victims' families.

But the world body acknowledged on Monday that raising enough funding would be difficult.

Alston has concluded that scientific evidence "points overwhelmingly to the responsibility of the peacekeeping mission as the source of the outbreak."

"The U.N. says, 'We don't know how it happened,'" he said.

"I had had a hope that this secretary-general would end his term by issuing a genuine and serious apology, not talking about moral responsibility, or other words that legal advisers come up with to avoid a reference to real responsibility," Alston said.

Ban is due to step down at the end of 2016 after serving two five-year terms.