International donors are threatening to withdraw funding for Uganda's South Sudanese refugee crisis over allegations that include misuse of relief items and the trafficking of girls and women, a government official said Tuesday.

The allegations threaten Uganda's global reputation as a progressive host to refugees fleeing violence in neighboring countries. The East African nation has struggled to support more than 1 million South Sudanese refugees, many of whom poured in over the past year and a half.

A criminal investigation is underway after the United Nations resident coordinator raised concerns last month, a minister in charge of refugees, Musa Ecweru, said in a statement.

The allegations of "gross mismanagement, fraud and corruption" include theft of relief items for refugees, he said. It was not clear what amount of aid allegedly was diverted. "It is important that these investigations are conducted in an independent manner," Ecweru said.

A U.N.-backed fundraising summit in Uganda in June brought in over $350 million for the refugee crisis.

The European Union, a main benefactor, called the allegations "troubling" and urged "urgent and thorough investigations as well as for changes to the registration system for refugees."

The EU's anti-fraud office has been notified and also will investigate the charges.

Official corruption is widespread in Uganda. Newspapers often report tales of officials accused of stealing government funds or using their offices for personal enrichment