An increase in new cholera patients in rural Haiti raises worries that the outbreak may be starting to surge again with the spring rainy season, a U.S.-based medical aid group said Wednesday.

Health experts in Haiti had warned in January when the cholera outbreak began to slow that there could be a surge of new cases in the spring as rains helped spread water contaminated with the cholera bacteria.

Workers for the aid group Partners in Health have noted a worrisome rise in new patients at treatment centers in Port-au-Prince and the countryside, spokeswoman Kathryn Mahoney said.

At centers in Mirebalais, a central town near where the outbreak was first detected in October, the number of new cholera patients has roughly tripled in recent weeks, Mahoney said.

However, Doctors Without Borders, which has had a leading role in responding to the cholera outbreak, is more optimistic.

The group's workers have seen a slight increase in new cases in the countryside but the overall number of cases in the country has been stable, said Sylvain Groulx, the group's chief of mission in Haiti.

"These are little, little spikes," Groulx said. "We're not expecting to see a second peak."

The cholera outbreak has now sickened 250,000 people in Haiti and killed nearly 5,000.

The United Nations was expected to disclose this week the findings of an independent investigation into the source of the outbreak. U.N. Secretary-General Ban Ki-moon requested the study amid reports of poor sanitation at a U.N. base near Mirebalais housing peacekeepers from Nepal.

The U.S. Centers for Disease Control and Prevention has said the strain of cholera found in Haiti matched one in South Asia, a region that includes Nepal.