The U.N. peacekeeping chief said Tuesday there is a chance for peace in South Sudan, with more progress since the warring sides signed a new peace deal in September than at any time since the conflict erupted five years ago.

But Jean-Pierre Lacroix told the U.N. Security Council that "the peace process is not yet assessed as fully sustainable and irreversible and will need positive engagement and compromise from the parties if it is to deliver genuine hope and relief to the suffering South Sudanese populations."

While there has been "a significant improvement in the general security situation of the country" since the peace agreement was signed, sporadic clashes have taken place "indicating that the improvements remain fragile," Lacroix said.

He said the responsibility to sustain "momentum" in implementing the agreement "lies solely with the parties."

There were high hopes South Sudan would have peace and stability after its independence from neighboring Sudan in 2011. But it plunged into ethnic violence in December 2013 when forces loyal to President Salva Kiir, a Dinka, started battling those loyal to Riek Machar, his former vice president who is a Nuer.

A peace deal signed in August 2015 didn't stop the fighting, and neither did a cessation of hostilities agreement in December 2017 nor a truce declaration on June 27. The Sept. 12 power-sharing agreement signed in neighboring Sudan, while fraught with delays, missed deadlines and some fighting, offers hope that the war that has killed nearly 400,000 people could end.

Moving forward, Lacroix said, the U.N. strongly believes two critical benchmarks must be given top priority in the pre-transition period under the peace deal, which ends in May 2019: a comprehensive agreement on the security sector and transitional security arrangements, and a new chair of the commission that will shepherd pre-transition negotiations.

In the course of South Sudan's conflict, sexual violence has been widely used as a weapon of war, and Lacroix decried the "horrific" mass rapes that took place in Bentiu in November.

The medical charity Doctors Without Borders reported that 125 women and girls had been raped, whipped and clubbed over 10 days while heading to a food distribution site in a dramatic spike in sexual violence. Lacroix said the U.N. peacekeeping mission urged South Sudanese authorities to protect women and girls in the area. He told the council that the mission's human rights team has launched an investigation to identify the perpetrators.

Pramila Patten, the U.N. special representative on sexual violence in conflict, called the attacks "part of a systematic trend and pattern of sexual violence that has escalated dramatically in 2018 despite recent re-commitments by South Sudan's leaders to a cessation of hostilities and a revitalized peace agreement."

She told the council the number of victims in 2018 "has already reached 1,157, making it the highest number recorded in the last three years." By comparison, she said, in 2017 there were 128 women and 68 girls who suffered from sexual violence.

Patten noted government ministers and officials had called the high prevalence of sexual violence "unacceptable and shameful," but she said there must be action.

"A policy of 'zero tolerance' cannot be underpinned by the reality of 'zero consequence,'" she said.

Patten urged the Security Council to raise the cost and consequences "for committing, commanding or condoning these crimes," including by imposing sanctions.

She said her office, together with the Office of the High Commissioner for Human Rights, have submitted a letter to the Security Council committee monitoring sanctions against South Sudan with the names of three alleged perpetrators from Southern Unity state. Bentiu was the state's capital before an administrative reorganization.