Some 14,000 South Sudanese, mostly women and children, have fled their country's civil war to Sudan in the past fortnight, creating an "emergency within an emergency," the UN refugee agency warned Saturday.
South Sudan descended into civil war in December 2013 when President Salva Kiir accused his ex-deputy Riek Machar of attempting a coup, splitting the country along tribal and ethnic lines.
More than 157,000 South Sudanese have fled to Sudan since the war started.
"In the last week we have an influx of about 7,000, and in the last two weeks about 14,000 arriving and that's considered an emergency within an emergency," said Ann Encontre, the UNHCR's regional refugee coordinator for South Sudan.
Most are from South Sudan's oil-producing Upper Nile and Unity states, the scene of intense fighting in recent weeks, and more than 80 percent of all South Sudanese who have fled to Sudan are women and children.
The influx has seriously strained UN agencies in the country, which are facing a severe funding shortfall.
They had hoped to raise $152 million (135 million euros) for 2015 to care for South Sudanese refugees in Sudan but have only received 10 percent of this.
The UN is also racing to prepare facilities for the new arrivals in Sudan's White Nile state ahead of the rainy season, which is due to start in the coming weeks.
"If we are not prepared, the rainy season could be disastrous, there could be outbreaks of water-borne diseases such as dysentery and cholera," Encontre said.
South Sudan seceded from Sudan in 2011 under a peace deal that ended a 22-year civil war.
Khartoum does not designate South Sudanese fleeing the conflict as refugees but grants them a special status allowing them access to health care, education and other services.
Encontre said she expected more South Sudanese to flee to Sudan before the rainy season, unless there is a breakthrough in negotiations to end the fighting.
Peace talks, led by regional bloc IGAD, have been going on in Ethiopia almost as long as the 18-month war, resulting in nine failed agreements and ceasefires.
U.N.: 14,000 S.Sudanese flee to Sudan in two weeks