Haiti is facing a devastating return of the cholera outbreak it has spent years fighting off following the havoc wrought by Hurricane Matthew.

The United Nations said this week that a current cholera outbreak may be underestimated because officials have not been able to monitor it in remote areas cut off from main cities, according to Reuters.

The UN also launched a new trust fund to help battle cholera and create clean water, sanitation, and healthy systems, though a U.N. special advisor to the Secretary-General, David Nabarro, said that the fund still needs more donations.

Read More: How You Can Help Hurricane Matthew Victims in Haiti

The storm, which killed more than 500 people when it hit Haiti on Oct. 4, destroyed roads and bridges, making it difficult for aid workers to reach some affected areas.

Haiti faced a cholera epidemic that killed more than 9,000 people and infected more than 800,000 following the 2010 earthquake that devastated the country. The disease was actually brought to the island by UN peacekeepers accidentally, the UN said earlier this year.

"We don't know if there are many people with the problem of cholera in the areas that we cannot access and that is why I ask the people, let us access everywhere," said David Nabarro, a special advisor to the U.N. Secretary-General. "We fear that there are people in caves, in other places, without help and they are perhaps sick.”

Read More: ‘People May Starve’: Hurricane Matthew Leaves Thousands Without Food, Water in Haiti

The UN announced it would launch a new plan to improve its cholera response in Haiti by creating rapid access to care and treatment for those infected. The World Health Organization said last week it would send 1 million cholera vaccines to the country, Reuters said.

The U.N. received only $15 million out of the $120 million it requested for Haiti relief following the hurricane, and Nabarro said he would ask for more from donors this week, according to the report.

The UN estimated that 1.4 million Haitians need urgent lifesaving assistance in the wake of the hurricane.