The International Committee of the Red Cross voiced alarm at reports of hundreds of civilian deaths across several regions of Syria, and the recent destruction of numerous hospitals and schools.
"For the past two weeks, we have seen an increasingly worrying spike in military operations that correlates with high levels of civilian casualties," head of ICRC's delegation in Syria, Marianne Gasser, said in a statement.
Violence has surged not only in places like Raqa and Deir Ezzor, where brutal battles are underway to oust the Islamic State group, but also in many so called de-escalation areas, like Idlib, rural Hama and Eastern Ghouta.
But Gasser warned that "the return to violence is once again bringing intolerable levels of suffering to wide areas of the country, while at the same time decreasing access for humanitarian agencies."
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"Taken together, these are the worst levels of violence since the battle for Aleppo in 2016," ICRC said.
It pointed out that some camps around Raqa and Deir Ezzor were receiving more than 1,000 civilians every single day, as men, women, children flee the bombings and battles.
And in the past 10 days alone, as many as 10 hospitals had been damaged across Syria, "cutting hundreds of thousands of people from access to even the most basic healthcare," it said.
Robert Mardini, who heads ICRC's Near and Middle East operations, stressed that "military operations must not disregard the fate of civilians and of the vital infrastructure on which their survival depends."
"Winning by any means is not only unlawful, but also unacceptable when it comes at such human cost," he said, urging all sides fighting in Syria to "show restraint, and to abide by the basic tenets of International humanitarian law."