Rebel forces launched an offensive Tuesday to retake the town of Malakal, the capital of a strategic oil-producing region in the world’s newest country, as reports emerged of a ferry accident in which as many as 200 South Sudanese fleeing clashes drowned, according to a military spokesman.

The deaths were the single deadliest incident reported in the nearly four-week-long conflict, which has raged on even as the warring sides are engaged in peace talks in Ethiopia with U.S. and African mediators. The boat capsized over the weekend when scores of people apparently jumped on board to cross the White Nile River and escape the rebel advance, said Col. Philip Aguer, the military spokesman.

“They were civilians, mostly women and children,” he said. “Only two survived.”

It was not possible to independently verify the report. In a press briefing in Geneva, a U.N. refugee agency official, Adrian Edwards, said no additional information was available.

The United Nations announced Tuesday that the humanitarian crisis in South Sudan has “deepened,” with more than 400,000 people displaced inside the country, while another 73,000 have fled to neighboring countries. The number of people seeking refuge at U.N. peacekeeping bases has risen to 66,500, with large numbers arriving at the base in Malakal over the past two days, it said.

“Today there is fighting anew in & around #Malakal, and the number of civilians seeking #UN protection has soared from 10,000 to 19,000,” Toby Lanzer, the U.N. deputy special representative and humanitarian coordinator for South Sudan, wrote in a message on Twitter.

The conflict was triggered last month by a political struggle between South Sudanese President Salva Kiir and his former deputy, Riek Machar. The fighting has divided the military and has sparked ethnically based attacks that have put the country on the brink of civil war. The United Nations has said hundreds have been killed; the International Crisis Group, a think tank, said last week that the number of deaths was closer to 10,000.

Fighting also raged outside Bor, 120 miles north of the capital, Juba, as government forces tried to push the rebels out of town, according to Aguer. Over the weekend, the government recaptured Bentiu, the capital of oil-rich Unity state. But the assault on Malakal underscored the rebels’ continued ability to carry out attacks.

There was little sign of a progress in the peace talks Tuesday. On Monday, the negotiations were reportedly broken up after delegates complained when the venue was shifted to a nightclub in a luxury hotel in the Ethiopian capital, Addis Ababa.

A senior U.N. human rights official, Ivan Simonovic, was expected in Juba on Tuesday to assess human rights conditions. Witnesses and U.N. officials have reported severe human rights violations by both warring sides, including ethnic massacres, summary executions and widespread looting.

On Sunday, the Satellite Sentinel Project, a human rights group, released images that showed destruction of civilian homes and market areas in two South Sudanese towns, Mayom and Bor, by government and rebel forces.

“Evidence of atrocities against civilians should be collected and used for future prosecution for war crimes,” the actor George Clooney, a co-founder of the group, said in a statement. “There will be no peace if massive human rights abuses can be committed with no accountability.”