More than one million refugees have fled South Sudan’s ongoing civil war, overwhelming aid agencies and creating one of the world’s worst humanitarian disasters.

Most of them are women and children who include survivors of violent attacks, sexual assault, children that have been separated from their parents or traveled alone, the disabled, the elderly and people in need of urgent medical care.

The United Nations said Friday that South Sudan joins Syria, Afghanistan and Somalia as countries that have produced over one million refugees.

“This is a very sad milestone,” said Leo Dobbs, a spokesman for the U.N. refugee agency. South Sudan’s estimated population was over 12 million last year, according to the World Bank.

Charlie Yaxley, a Spokesperson of the United Nations High Commission for Refugees-UNHCR says that many arrive exhausted after days walking in the bush and going without food or water. Many children have lost one or both of their parents, some forced to become primary caregivers to younger siblings.

South Sudan gained independence from Sudan in 2011, but civil war erupted two years later and tens of thousands have been killed. New fighting in July in the capital, Juba, created a surge of more than 185,000 refugees. Most people fleeing are women and children.

Neighboring Uganda hosts the highest number of refugees, and 20,000 have arrived in the past week due to clashes in southern South Sudan. Ethiopia, Kenya, Sudan, Congo and Central African Republic also have received tens of thousands of people fleeing.