The internal conflict in South Sudan has forced more than 1 million people out of the country, the UN Refugee Agency (UNCHR) said Friday.
The African nation joins Syria, Afghanistan and Somalia as countries that have generated more than 1 million refugees.
Some 185,000 people have fled since early July when violence again flared in the capital Juba.
In addition to the refugees, 1.61 million people have been internally displaced by the violent power struggle between President Salva Kiir and his former deputy Riek Machar that erupted in late 2013.
UNHCR spokesman Leo Dobbs said that "humanitarian organizations are finding it very difficult for logistical, security and funding reasons to provide urgent protection and assistance to the hundreds of thousands in need."
Most of those who leave the country are women and children, including survivors of violence, sexual assault and children who have become separated from their parents, he said.
Uganda is hosting some 374,000 South Sudanese refugees. A third of them have arrived since July.
An estimated 292,000 have fled to Ethiopia, while Sudan has taken in 247,000 of its southern neighbours.
The two sides of the conflict formed a unity government in April, and Machar was reinstated as vice president.
But fighting erupted again in July, prompting Machar to flee to Sudan.