The U.N. humanitarian office says more than 50,000 South Sudanese have fled into the troubled Darfur region in neighboring Sudan this year.
OCHA's statement on Thursday highlights the desperate humanitarian situation in South Sudan, where a civil war began in December 2013 and a peace deal has been violated repeatedly.
OCHA says South Sudan refugees started entering East Darfur in January, driven also by deteriorating food security.
Darfur has been in turmoil since 2003. The area South Sudanese have fled to has not seen the worst violence.
Also on Thursday, Doctors Without Borders warned that fighting in South Sudan has stopped thousands of people from accessing medical and humanitarian aid.
The group says two of its clinics were destroyed during fighting in the Greater Upper Nile region four weeks ago.