Thousands of women fleeing the four-year-long conflict in South Sudan have been raped and sexually assaulted, according to rights groups and women who have spoken to Al Jazeera.

They mainly blame the government troops of President Salva Kiir, but also opposition forces loyal to former Vice President Riek Machar for the abuses.

Many women who survived described how their husbands were killed before they were gang-raped by government soldiers.

"My husband was following a short distance behind us," one rape survivor told Al Jazeera.

"When he came and found these men on me, he told them to stop," she said as she described how five government soldiers gang-raped her, along with four other women.

"They grabbed him immediately and killed him with a knife."

Al Jazeera heard similar stories from other women now living refugee camps in Uganda.

"They tied a blindfold on my face," another rape survivor said. "They took all of my possession and stripped me," she added.

"Three of them were all on me. After, I grabbed my baby and left naked. Now, I have nothing."

War crimes

Ken Scott, a war crimes prosecutor who has worked on tribunals for many conflicts, told Al Jazeera that the sexual violence in South Sudan is the worst he has ever seen.

"It's such a high level of incidents, widespread, been going on for a substantial period of time, not isolated incidents, [and] one can only conclude that war crimes involving sexual violence are taking place," Scott said.

A spokesman for government forces told Al Jazeera that soldiers who rape are punished. He also questioned the stories coming from the refugee camps.

"How do we substantiate those claims, to know for sure they are not made up without someone coming forward to report it?" Lul Ruai Koan, a government army (SPLA) spokesman, told Al Jazeera.

"These are claims that are being made in the refugee camps," he said.

"How do we get convinced they are not being told to say weird things about the SPLA?"

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But rights groups say they have documented widespread sexual violence in South Sudan.

A report by Amnesty International in July found that "thousands of South Sudanese has been subjected to sexual violence including rape, gang rape, sexual slavery, sexual mutilation, torture, castration, or forced nudity."

The report found that the perpetrators were from all sides of the conflict.

"This is pre-meditated sexual violence on a massive scale," Munthoni Wanyeki, Amnesty's regional director for East Africa, the Horn and the Great Lakes, said in a statement during the report's release.

"Women have been gang-raped, sexually assaulted with sticks and mutilated with knives," Wanyeki said.

Amnesty called on the government to "take deliberate measures to halt this epidemic of sexual violence".

After gaining independence from Sudan in 2011, South Sudan descended into war in December 2013, pitting President Salva Kiir's troops against those or rebel leader and former Vice President Riek Machar.

A peace accord was signed in August 2015 and Machar returned to the capital in April last year to share power with Kiir, before the deal fell apart less than three months later and Machar and his supporters fled the capital.

Tens of thousands of people have been killed and more than 3.5 million have been displaced since the conflict began.

The war has created what has been called one of the world's fastest-growing refugee crises and both sides of the conflict have been accused of abuses.

Al Jazeera's Malcolm Webb reports from West Nile, Uganda.