Victims of Haiti's cholera epidemic have given the United Nations a 60-day deadline to start talks about billions of dollars worth of compensation or face legal action.
The UN is accused of negligently allowing peacekeeping soldiers to pollute Haiti's water with cholera.
A UN cholera expert agrees that this is "most likely" to be true.
The UN rejected an earlier call for compensation and continues to insist it is immune from legal proceedings.
The cholera epidemic began in Haiti in 2010 near a camp for UN soldiers, where there were leaking sewage pipes. Some human waste was also dumped near a river outside the camp.
The camp housed UN soldiers from Nepal, where cholera is endemic. The UN's own cholera expert, Danielle Lantagne, has said that Haiti's outbreak is likely to have come from UN soldiers.
The victims include relatives of the 8,000 people who have died and hundreds of thousands of people who have fallen sick.
Lawyers for the victims say the UN is breaking international law.
They say they will open legal proceedings in New York with claims totalling many billions of dollars if the UN does not start talks within 60 days.
The lawyers say they will file claims for $100,000 (£64,000) for the families of those who have died and $50,000 (£32,000) for every one of the hundreds of thousands who have fallen sick.
The UN's relative silence on the matter so far may be because it simply does not know what to do in the face of what could be a series of catastrophic and deadly errors, says the BBC's International Development Correspondent Mark Doyle.