Kampala: 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 says South Sudan joins Syria, Afghanistan, and Somalia as countries that have produced over one million refugees, as people continue to flee to neighbouring countries.

"This is a very sad milestone,'' said Leo Dobbs, a spokesman for the UN refugee agency.

South Sudan gained independence from Sudan in 2011, but civil war erupted two years later and tens of thousands have been killed.