UNITED NATIONS — Even as diplomats hail signs of progress marking the sixth anniversary of the conflict in Syria, grim findings underscore the bloody civil war’s toll on its youngest victims: Children are enduring “unprecedented suffering.”

In “Hitting Rock Bottom: How 2016 Became the Worst Year for Syria’s Children,” the UN Children’s Fund documents show that 652 children died in 2016 as a result of the fighting.

Last year surpassed 2015 by 20 percent as the deadliest year for children since 2014, when officials began formally verifying deaths. Of those children killed in 2016, 255 lost their lives in or near schools — among the civilian institutions that laws of war say are to be shielded from attack. Further, some 647 children were maimed in 2016, up 25 percent from 2015, according to the report.

“The depth of suffering is unprecedented,” said Geert Cappelaere, UNICEF regional director for the Middle East and North Africa, on Monday. “Millions of children in Syria come under attack on a daily basis, their lives turned upside down. Each and every child is scarred for life, with horrific consequences on their health, well-being and future.”

The numbers, UN officials said, are among the worst since the conflict began in March 2011.

More than 850 children were recruited as soldiers in 2016, twice the number documented in the previous year, according to UNICEF researchers in the 12-page report, and some were used “in extreme cases as executioners, suicide bombers or prison guards.”

The report documents 87 attacks on schools in 2016, and 1.7 million Syrian children, as well as 530,000 refugee children, out of school. Over 2.3 million Syrian children have migrated to Turkey, Lebanon, Jordan, Egypt and Iraq. Others have gone to European countries, the report said.

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A Rand corporation scholar, Shelly Culbertson, said that based on a study she conducted in 2015, between 50 and 60 percent of school-age Syrian refugees in Turkey, Jordan and Lebanon — or 700,000 — were not attending school.

The Office of the UN High Commissioner for Refugees notes that about 5 million Syrians have left the country since March 2011 and are registered in Turkey, which now hosts 2.9 million. Jordan, Lebanon, Egypt and Iraq accepted 2 million, and another 29,000 have gone to North Africa.

The report comes as UN Secretary-General Antonio Guterres’ special envoy to Syria, Staffan de Mistura, prepared to reopen talks aimed at ending the war that the UN said has claimed at least 400,000 lives.

The envoy struck an optimistic tone Wednesday when he spoke about nine days of peace talks that concluded March 3 in Geneva. A fifth round of talks is expected to take place in Geneva as early as next week.

De Mistura’s optimism dovetails with the apparent hold of a cease-fire among the warring parties and a general improvement in the humanitarian situation. Talks held last year broke down as a fragile cessation of hostilities shattered and the war intensified.

In what may be another sign of progress, The Associated Press reported that Syrian opposition fighters will be allowed to leave the last rebel-held neighborhood in Homs under a Russia-backed deal signed on Monday. Rebel fighters who stay in the city would be entitled to a government amnesty.