The aid agency MSF says it has evacuated 42 of its staff from the Dadaab refugee camp for Somalis in northeastern Kenya, citing "deteriorating security conditions".
Al-Shabab, the Somalia-based al-Qaeda-linked group, has carried out deadly raids in villages in northern Kenya in recent weeks.
"The current security situation is severely limiting the ability of our medical staff to provide humanitarian aid to people who desperately need it," Charles Gaudry, head of the Kenya mission of MSF, or Doctors Without Borders, said on Thursday.
"Refugees and medical staff are bearing the brunt of the deteriorating security conditions," he said.
Kenya's government has hosted refugees from Somalia since 1991 when civil war tore the country apart.
Since then Dadaab has grown into the world's largest refugee settlement, with over 350,000 residents.
MSF said "the precautionary measure" has had a direct impact on its ability to provide much-needed medical care to refugees, with two out of its four health posts closed.
MSF called on "armed groups to guarantee respect for medical facilities, patients and staff."
In April al-Shabab fighters attacked a university in the nearby town of Garissa, killing 148 people including 142 students.
Last week al-Shabab ambushed a police patrol in the village of Yumbis, about 70km north of Garissa, killing at least one officer and injuring several.