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.

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 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.

The U.N. praised the countries, some of the world's poorest, for allowing refugees to enter.

"Many refugees arrive exhausted after days walking in the bush and going without food or water," Dobbs said. "Many women and girls said they were sexually assaulted during their flight."

Another 1.6 million people are displaced inside South Sudan. The country's population was estimated at more than 12 million last year, according to the World Bank.

The fighting that erupted in July between supporters of President Salva Kiir and then-Vice President Riek Machar "has shattered hopes for a real breakthrough and triggered new waves of displacement and suffering," Dobbs said.

A peace deal reached a year ago continues to be violated. Machar fled the country during July's chaos.

South Sudan has been threatened by the U.N. Security Council with an arms embargo if it does not accept 4,000 additional peacekeepers to help protect civilians. The government calls the plan a violation of its sovereignty.