Around 110,000 people have fled to Uganda from South Sudan this year, most of them escaping fighting that erupted anew last month, the United Nations said Friday.
The UN's refugee agency said it was "extremely worried" about South Sudan's rapidly escalating displacement crisis, warning that neighbouring countries and humanitarian groups were struggling to meet demands.
Roughly 82,000 of those who have sought refuge across South Sudan's southern border in 2016 have moved in the last five weeks, UNHCR said.
The influx was sparked by renewed fighting between the government forces of President Salva Kiir and those loyal to ex-rebel chief Riek Machar.
Another 100,000 South Sudanese have fled north this year into Sudan, but most of that movement occurred earlier in the year and was partly linked to food insecurity.
Overall, more than 2.5 million people have been driven from their homes since South Sudan's war erupted in December 2013, including 930,000 who have fled to neighbouring countries, UNHCR said.
"What you are seeing in South Sudan is now the world's fourth biggest refugee situation" behind Syria, Afghanistan and Somalia, UNHCR spokesman Adrian Edwards told reporters in Geneva.
Those fleeing to Uganda in recent weeks -- 90 percent of whom are women and children -- have increasingly reported being targeted in robberies and sexual assaults, UNHCR said.
"Armed groups are also reportedly abducting children aged 12 and above from schools and threatening people," the UN agency added in a statement.
Uganda, which has long been a landing spot for South Sudanese refugees, is working to expand reception centres and camps near the border.
But some sites are already hosting five times the number of people they were designed accomodate.
UNHCR said its $609 million (546 million euros) response plan for South Sudan is only 20-percent funded.