(AFP) - The number of refugees from South Sudan has passed the one million mark after a renewed bout of fierce fighting in July sent nearly 200,000 people fleeing the war-scarred nation, the UN said Friday.

The latest United Nations refugee agency figures see the world's youngest nation join the ranks of Syria, Afghanistan and Somalia, where conflict similarly has driven massive numbers fleeing to safety across national borders.

The large majority of South Sudanese refugees registered since it won its independence in 2011 have fled since the outbreak of a particularly brutal civil war in December 2013.

Tens of thousands of people have died and more than 2.5 million been driven from their homes.

Countless villages have been burnt to the ground, almost half the population relies on food assistance to survive, and human rights organisations say government and rebel forces have frequently used rape as a weapon of war.

"The number of South Sudanese refugees sheltering in neighbouring countries has this week passed the one million mark," UNHCR said in a statement.

Another 1.61 million people are displaced inside the country, it said.

"Five years after independence, this is a very sad milestone," spokesman Leo Dobbs told reporters in Geneva.

Neighbouring Uganda, which already shelters 375,000 South Sudanese, warned it was running out of resources and asked for support.

"The international community ... must act very fast to end this violence," Disaster and Refugees Minister Musa Ecweru told AFP. "We have maintained an open door policy ... but the resources we have cannot cope with the surging numbers."

"We appeal to donors to step up funding," he added.

Hopes of ending the three-year conflict rose in April when former rebel leader Riek Machar returned to Juba to take up the job of vice-president in a national unity government headed by President Salva Kiir.

- No end in sight -

But fierce clashes erupted in Juba on July 8 between Kiir's guards and troops loyal to Machar, who is currently in Khartoum receiving medical treatment.

Since then, more than 185,000 people have fled, most of them women and children, according to the UNHCR.

"They include survivors of violent attacks, sexual assault, children (who) have been separated from their parents ... and people in need of urgent medical care," the UN agency said.

On September 4, the South Sudanese government first reluctantly agreed to the deployment of a 4,000-strong UN protection force to beef up the UN's peacekeeping mission of 12,000 troops.

But then it asked to re-negotiate the size of the force, irritating the international community.

On Thursday the United States threatened to push for an arms embargo against the Juba government should it block the formation of the force.

Meanwhile several civil society activists who met with a UN Security Council team last week have fled a government crackdown.

The UNHCR said refugees arriving in neighbouring countries were reporting heavy fighting across the southern Greater Equatoria region, where armed groups were killing civilians, sexually assaulting women and girls and recruiting young boys.

"Many refugees arrive exhausted after days walking in the bush and going without food or water," UNHCR said.

Most of those recently uprooted have crossed into Uganda, which counts 143,164 recent arrivals, bringing the total number of South Sudanese refugees in the country to nearly 375,000.

And there is no end in sight: over the past week alone, more than 20,000 new arrivals were registered in Uganda.

A surge of people has meanwhile also entered western Ethiopia's Gambella region in the past week, while others have headed to Kenya, Democratic Republic of the Congo and Central African Republic.

"These countries have commendably kept their doors open to the new arrivals," UNHCR said.

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