The number of South Sudanese refugees sheltering in Uganda has reached one million, the United Nations says, marking a grim milestone for what has become the world's fastest-growing refugee crisis.

Ugandan officials say they are overwhelmed by the flow of people fleeing South Sudan's civil war, and the UN refugee agency urges the international community to donate more for humanitarian assistance.

An average of 1,800 South Sudanese citizens have been arriving daily in Uganda over the past 12 months, the UNHCR said in a statement. Another one million or more South Sudanese are sheltering in Sudan, Ethiopia, Kenya, the Congo, and the Central African Republic.

The number of people fleeing jumped after deadly fighting again erupted in South Sudan's capital, Juba, in July 2016.

"Recent arrivals continue to speak of barbaric violence, with armed groups reportedly burning down houses with civilians inside, people being killed in front of family members, sexual assaults of women and girls, and kidnapping of boys for forced conscription," the statement said.

"With refugees still arriving in their thousands, the amount of aid we are able to deliver is increasingly falling short."

A fundraising summit hosted by Uganda in June raised only a fraction of the $2 billion that Ugandan officials have said is needed to sufficiently look after the refugees and the communities hosting them.

Most of the refugees are women and children fleeing violence, often along ethnic lines, since the world's newest country erupted into violence in December 2013.

Ugandan refugee officials have repeatedly warned the influx is straining the country's ability to be generous to the refugees, who often are given small plots of land for building temporary shelters and planting crops when they arrive.

The largest of the settlements hosting refugees from South Sudan, Bidi Bidi, is roughly 230 square kilometers.

The World Food Program cut food rations for some refugees amid funding shortages in June.

The UN says at least $674 million is needed to support South Sudanese refugees in Uganda this year, although only a fifth of that amount has been received.

The money is needed to provide basic services, including stocking clinics with medicines and putting up schools. Aid agencies say classroom sizes in the few available schools often exceed 200 pupils, and other children have dropped out because the nearest schools are located miles away.

"The funding shortfall in Uganda is now significantly impacting the abilities to deliver life-saving aid and key basic services," the UNHCR statement said.

Fighting persists in parts of South Sudan despite multiple ceasefire agreements. Rebel forces said Tuesday they had reclaimed their stronghold of Pagak in the northeast, less than a week after being pushed out by government forces.

Both sides have committed serious rights violations, including murder and rape, against civilians, according to UN investigators.

(Source: AP)