Lawyers representing victims of a cholera epidemic in Haiti have filed a lawsuit against the United Nations at a court in New York.
They say UN peacekeepers introduced cholera to Haiti in 2010. The disease killed more than 8,000 people and made hundreds of thousands sick.
The lawyers are demanding compensation of $100,000 (£62,000) for every person who died and $50,000 for each of those who became ill.
The UN says it has legal immunity.
Lawyers filed the suit at the US District Court in New York. They said they were left with no other option after the UN had rejected previous claims for compensation.
"The UN refused to even consider them. We then felt we had no choice but to file in a national court," Brian Concannon, director of the Institute for Justice and Democracy in Haiti, which is bringing the case, told the BBC.
BBC international development correspondent Mark Doyle says investigations have pointed strongly to leaking sewage at a camp for UN soldiers from Nepal, where cholera is endemic, as the origin of the outbreak in Haiti.
No cases of the bacterial infection, which causes diarrhoea, nausea, vomiting and muscle cramps, had been recorded in Haiti for a century until the outbreak in late 2010.
Then cases mounted quickly in an area near the camp.
Leading cholera expert Danielle Lantagne, who once worked for the UN, said in the past that the outbreak's "most likely source" was the UN camp.
In February, a spokesman for UN Secretary General Ban Ki-moon said the petition for compensation was "not receivable" under a 1947 convention which grants the UN immunity for its actions.
Our correspondent says the UN's position is unlikely to change.