The US Peace Corps announced Wednesday it was pulling hundreds of volunteers from Guinea, Liberia and Sierra Leone due to growing concerns over the spread of the deadly Ebola epidemic raging in West Africa.
Another US group, Christian charity Samaritan's Purse, said it was also temporarily withdrawing its non-essential staff from Liberia, citing regional "instability and ongoing security issues."
A Peace Corps spokeswoman said two of the group's volunteers had contracted Ebola after coming into contact with someone who later died from the virus, but she stressed that they were not symptomatic and were currently under medical observation and being held in isolation.
"When they receive medical clearance for return to the US, we will work with them to travel safely back," she added.
At least one American has died from the virus after contracting it in Liberia, State Department spokeswoman Marie Harf said. Two others have been infected.
The Peace Corps said it would "closely monitor" the outbreak with US experts from the Centers for Disease Control and Prevention and State Department before a future return to the three countries, those most affected by the outbreak. The US government-run volunteer agency set no date for resuming operations.
"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," it said in a statement.
Two Americans associate with Samaritan's Purse "have shown a slight improvement in the past 24 hours," but both remain in serious condition after contracting the virus in Liberia, the group said. It was unclear if they were the same two cases to which Harf referred.
The two Americans were identified as Kent Brantly, a doctor working for Samaritan's Purse, and Nancy Writebol, a missionary with partner organization Service in Mission (SIM).
There are currently 102 Peace Corps volunteers in Guinea working on agriculture, education and health, 108 in Liberia and 130 in Sierra Leone.
The three countries are struggling to contain an epidemic that has infected 1,200 people and left at least 672 dead across the region since the start of the year.
Fears that the outbreak could spread to other continents have been growing with European and Asian countries on alert and leading medical charity Doctors Without Borders (MSF) warning the epidemic is out of control.
The Peace Corps said it had provided volunteers in adjacent areas to those countries with guidance to take the "utmost precautions" to protect their health in recent months.
Ebola, which first emerged in 1976, is believed to be carried by animals hunted for meat, notably bats.
It spreads among humans via bodily fluids and has killed 56 percent of those it has infected in the current outbreak.
Explore further: Top Sierra Leone doctor dies of Ebola