Updated

Aid officials in South Sudan say tens of thousands of refugees fleeing war in Sudan are struggling to find enough water to drink and cook with, leading to the deaths of an unknown number from dehydration and diarrhea.

The U.N. said Monday that more than 35,000 people have fled Sudan's Blue Nile state for South Sudan over the last month. The refugees said they fled bombing and ground fighting between Sudan's military and a rebel group historically associated with South Sudan.

Tara Newell with the aid group Doctors Without Borders said thousands of the new arrivals were forced to walk about 13 miles over the last week in search of water. Newell said an unknown number have died.

South Sudan gained independence from Sudan in July 2011 after decades of civil war.