The Syrian government has unleashed an aggressive bombardment on a rebel-held suburb near Damascus, killing an estimated 250 people as it steps up a siege on the last major rebel stronghold.

The U.K.-based monitoring group, the Syrian Observatory for Human Rights, said it is the deadliest 48 hours in eastern Ghouta since a 2013 chemical attack, according to the BBC. More than 50 children have died in airstrikes and artillery fire, and 106 civilians were killed in the enclave on Tuesday alone, Reuters reports.

The U.N. has warned that the situation is “spiraling out of control” and called for a ceasefire to evacuate the wounded and deliver medical aid. A U.N. spokesperson said at least six hospitals had been deliberately targeted, and that such attacks may amount to war crimes, the BBC reported.

The Syrian military said it had responded to mortar fire from rebels in eastern Ghouta with “precision strikes” on fortified positions.

About 400,000 people are trapped in eastern Ghouta, an area that has been under government siege for years. The Syrian government claims that the only civilians who remain in the enclave are being used as human shields by the rebels, the New York Times reports.

Just one humanitarian convoy has been permitted to enter the suburb since late November, and there have been reports of severe food shortages and child malnourishment.

On Tuesday, the U.N. children’s fund UNICEF issued a “blank statement” saying “we no longer have the words to describe children’s suffering and our outrage.”

In the north, pro-Syrian government forces entered the Kurdish enclave of Afrin to help fend off an assault by Turkish troops who are trying to push back the Kurdish YPG militia into northern Syria.

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Write to Laignee Barron at Laignee.Barron@time.com.