The UN has taken the rare step of invoking its legal immunity to rebuff claims for compensation from 5,000 victims of the Haiti cholera epidemic, the worst outbreak of the disease in modern times and widely believed to have been caused by UN peacekeepers importing the infection into the country.

Citing a convention laid down in 1946, the UN secretary general, Ban Ki-moon, telephoned President Michel Martelly of Haiti to tell him that the UN was not willing to compensate any of the claimants. The epidemic has killed almost 8,000 people and stricken hundreds of thousands more – about one out of every 16 Haitians.

For the UN to claim immunity for a crisis that most experts are convinced it unwittingly caused through its own disaster relief mission is highly contentious. The infection is thought to have been carried into Haiti by UN peacekeepers from Nepal sent to help with disaster relief following the 2010 Haiti earthquake.

Ban's spokesperson issued a carefully worded statement that pointedly did not accept or deny liability for the epidemic. But the statement made clear that the UN would not countenance the compensation claims, invoking its immunity from such legal disputes under section 29 of the Convention on the Privileges and Immunities of the UN.

The convention was one of the first treaties passed by the UN at its inception in 1946. In it, the UN grants to itself legal protections in the countries in which it operates, while section 29 extends that immunity to any UN worker operating in an official capacity.

This is not the first time that the UN has invoked its own immunity, but it is a highly unusual move made more controversial by the extreme distress in Haiti to which it relates. Sensitivities are running high within the UN headquarters in New York.

The suit for compensation was brought in November 2011 by the Institute for Justice and Democracy in Haiti (IJDH), a group of lawyers based in Boston. It sought to require the UN to install a national water and sanitation system to control the epidemic, pay compensation to victims for their losses, and make a public apology for its "wrongful acts".

Nicole Philips, speaking in IJDH's office in Haiti, gave a jaundiced reaction to the announcement, saying they were "not surprised by this at all. It's all very political". She said almost three times as many people had died in the continuing crisis as in the terrorist attacks of 9/11.

"If this had been a corporation, and if it had been an environmental spill, there would have been liability," Philips said.

Mario Joseph, the lead counsel for the 5,000 victims on whose behalf IJDH filed the lawsuit, plans to travel from Port-au-Prince to the worst-affected Artibonite department to pass on news of Thursday's UN statement.

"The United Nations can't have humanity and impunity at the same time. [The victims] still do not know what the UN has said, as they live in the countryside, but I know that they will want us to fight for justice from the UN."

Alix MacGuffie, 42, who lives in the coastal port town of Saint Marc in western Haiti's worst-affected Artibonite department, got cholera in July 2011. Speaking on the phone, he responded to news of the UN's decision. "I could have died from cholera. The UN caused us much harm and we should get compensation. If we don't stop cholera, what will happen in the future?"

Maximilien Saint Juste, 29, an electrician at a hospital in Saint Marc, said he was treated for cholera in November 2011 at the very hospital at which he works and though he is better now, he faces "discrimination" for having had the disease. "I think every victim should get compensation. The UN hasn't taken responsibility for the fact that they brought cholera to Haiti. Many people died from cholera and many of us suffer discrimination. A lot of people think that cholera cannot be treated entirely; they believe that cholera always sticks in your blood cells."

Cholera broke out in Haiti in Octo­ber 2010. Since then there has been substantial medical research, including full genome sequencing on the strain of cholera found in Haiti, which identified UN peacekeepers from Nepal as the source of the disease.

In October 2012, two years after the outbreak, a leading US cholera specialist, Dr Daniele Lantagne, said, "that the most likely source of the introduction of cholera into Haiti was someone infected with the Nepal strain of cholera and associated with the UN Mirebalais camp." She said "that the strain of cholera in Haiti is an exact match for the strain of cholera in Nepal."

The UN has accepted that inadequate sanitation at its barracks in Mirebalais, 60km north-east of the capital, Port-au-Prince, might have been a possible source of the bacterium. But it has doggedly refused to accept liability, insisting that the source of the cholera remains unclear and that the epidemic was caused by a "confluence of circumstances" that included problems with water, sanitation and public health.

While rebuffing the compensation claims, the UN has vowed to continue its efforts to contain the epidemic. So far the UN has spent $118m on medical equipment, health networks, water and sewerage improvements, health education at schools and other programmes designed to stem the crisis.

In December, Ban launched an initiative to eliminate the infection, and a new oral vaccination campaign is in the offing.