BEIRUT (AP) — The head of the U.N. Commission of Inquiry on Syria said Tuesday that fighting in the country has reached its worst point in years, with devastating consequences for civilians.

“We are witnessing the largest escalation of hostilities in Syria in four years,” Paulo Pinheiro told the U.N. General Assembly. “Yet again there appears to be total disregard for civilians’ lives in what are often tit-for-tat reprisals.”

A drone strike earlier this month on the Homs Military Academy killed 89 people, including 31 women and five children, and wounded as many as 277. No group claimed responsibility for the attack but the Syrian military accused insurgents “backed by known international forces” of carrying it out and launched a brutal campaign of airstrikes on opposition-held areas of northwest Syria in retaliation.

“In just four days of ground shelling... some 200 civilians were killed and injured, and medical facilities, schools and markets were impacted yet again,” Pinheiro said. “Tens of thousands are again displaced and on the run.”

Meanwhile, in retaliation for an attack in Ankara that injured two members of Turkish security forces, Turkey pounded Kurdish-controlled areas in northeast Syria that it said were used by the Kurdistan Workers’ Party, or PKK — a Kurdish separatist group that has waged a decadeslong insurgency in Turkey — as well as its allies in Syria in the People’s Defense Units, or YPG.

The Turkish bombing resulted in “destroying and damaging power and water stations, affecting hundreds of thousands of civilians,” Pinheiro said.

He also pointed to recent airstrikes on the Damascus and Aleppo airports in government-held Syria, reportedly by Israel that put both out of commission and “may have yet again impacted the delivery of humanitarian aid.”

Syria’s uprising-turned civil war, now in its 13th year, has killed nearly half a million people, displaced half of its prewar population of 23 million and crippled infrastructure in both government and opposition-held areas.