The Peace Corps is removing more than 300 of its volunteers from Sierra Leone, Liberia and Guinea as an Ebola outbreak that has killed hundreds worsens in West Africa.

In a statement issued Wednesday, the Peace Corps said it is temporarily removing volunteers from the affected countries. It did not offer a possible return date.

The news came just days after two American charity workers contracted the disease in Monrovia, the Liberian capital. Patrick Sawyer, a 40-year-old consultant with the Liberian government, was also revealed to be the first American to die of the disease, relatives said Tuesday.

“The Peace Corps has enjoyed long partnerships with the government and people of Liberia, Sierra Leone and Guinea, and is committed to continuing volunteers’ work there,” the statement read. “A determination on when volunteers can return will be made at a later date.”

The Peace Corps has 102 volunteers in Guinea, 108 in Liberia and 130 in Sierra Leone, according to the statement.

The outbreak has claimed at least 672 lives and has spread to more than 60 locations in Africa, making it the deadliest Ebola outbreak in history, according to the World Health Organization.

Government officials have become increasingly concerned about the virus’ rapid spread in recent weeks.

Sawyer, the American victim, was supposed to fly back to see his family in Minnesota next month. He died in Lagos, Nigeria, shortly after taking a flight from Monrovia, officials have said.

Dr. Kent Brantly and Nancy Writebol, the two American charity workers who contracted Ebola in Monrovia, were listed in serious condition on Wednesday, according to a statement issued by Samaritan’s Purse, the Catholic charity they were affiliated with.

Although their situations remained dire, both had shown “slight improvement” in the last 24 hours, according to the charity.

“We are doing everything possible to help Dr. Brantly and Nancy,” Samaritan’s Purse President Franklin Graham said. “We ask everyone to please pray urgently for them and their families.”

Citing “instability and ongoing security issues” Samaritan’s Purse also announced plans to remove many of its personnel from Liberia on Wednesday. However, medical staff will remain in the area to treat patients, the statement said.

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