Story highlights Eyewitness: "The jets arrived over Idlib and we heard three explosions"

At least 28 children are among the dead

The US and Russia agreed to a ceasefire plan that starts Monday

(CNN) At least 90 people were killed in airstrikes that rained down on northwestern Syria on Saturday and Sunday, a rights group said. The surge in violence came just hours after the US and Russia announced a new ceasefire plan.

The airstrikes landed in the rebel-held areas of Idlib in the northwest and Aleppo in the north of the country, according to the monitoring group Syrian Observatory for Human Rights.

The worst strikes were in Idlib, where at least 61 people were killed and more than 100 wounded in an attack that targeted a crowded market, an activist who witnessed the bombing and its aftermath told CNN.

The activist, who asked to be identified as Omar for security reasons, said he was in Idlib to cover the atmosphere a day ahead of the Eid al-Adha holiday. Omar took a detour to the city since the roads near the markets were closed as a security precaution, and that probably saved his life, he said.

"We heard a whistling sound then the explosions," Omar said. The jets arrived around 12:30 p.m. local time, he said. "Two or three rockets landed in middle of the market. We left the car where we were and headed towards the site of explosion."

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