The petition demanding compensation was filed last year on behalf of a sample group of five Haitians and said at least 8,300 people had died from cholera and a further 679,000 others had fallen ill since the outbreak began in October 2010.
There had been no cholera in Haiti for at least 150 years until it was allegedly introduced to the Caribbean nation by Nepalese UN peacekeepers sent there in the wake of the devastating earthquake in January 2010.
The brief, which was filed by the Department of Justice, had met a yesterday deadline as the New York district court decides whether the case can proceed.
Expressing sympathy for the victims over "the tragic loss of life" due to the cholera outbreak, Psaki said the US government did not lack empathy.
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But "the United States has legally binding treaty obligations that require it to afford the UN immunity from suit and also provide immunity of UN officials," she told reporters.
The strain of cholera is the same as one endemic in Nepal.
"The outbreak resulted from the negligent, reckless, and tortuous conduct of the defendants," the petition says.
It further alleged that the UN failed to "exercise due care to prevent the devastating outbreak of such disease," even though it knew that Haiti's weak water and sanitation made it vulnerable to waterborne diseases.
The Institute for Justice and Democracy in Haiti, which brought the petition on behalf of the victims, appealed to US Secretary of State John Kerry in a letter Tuesday "to do everything in his power to ensure that victims of Haiti's ongoing cholera epidemic have their day in court and can access justice.