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The coalition targets Raqqa in a series of airstrikes over the weekend

An envoy calls them "the most sustained airstrikes to date" against ISIS in Syria

CNN  — 

A wave of coalition airstrikes took aim at ISIS targets in the group’s Syrian stronghold of Raqqa over the weekend.

The latest series of attacks were part of “the most sustained set of airstrikes to date” against ISIS in Syria, Brett McGurk, the U.S. envoy for the coalition against ISIS, said on Twitter.

Near Raqqa, 18 airstrikes destroyed ISIS vehicles and bridges Saturday, the coalition said.

At least 23 ISIS militants were killed in the airstrikes in the Raqqa area, according to the London-based Syrian Observatory for Human Rights. One strike targeting an ISIS member in a school area also killed six civilians, the observatory said. A spokesman said the coalition “is unaware of any confirmation of civilian casualties.”

The goal, according to Brig. Gen. Kevin Killea, is to deny the militants’ “freedom to maneuver” and help ground forces strike back against ISIS.

Extremists have made Raqqa, which sits on the banks of the Euphrates River, the de facto capital of their self-declared Islamic State, which stretches across large areas of Syria and Iraq.

And the U.S. effort to take out senior ISIS leader Abu Bakr al-Baghdadi and other top ISIS officials has been increasingly focusing on the city.

ISIS capital in Syria is ‘like a big prison,’ activist says

CNN’s Barbara Starr, Merieme Arif and Eliott C. McLaughlin contributed to this report.