Warplanes from a U.S.-led coalition have conducted at least 150 airstrikes on the Islamic State group to bolster a major offensive on the extremists' Syrian stronghold of Raqqa, a monitor said Friday, May 27, The Daily Star said citing AFP.

The U.S. is backing twin assaults against IS-- one for Raqqa province, and one for the Iraqi city of Fallujah across the border.

A Kurdish-Arab alliance is being supported by coalition raids as well as U.S. forces on the ground in its push for territory north of Raqqa city -- IS' bolthole in Syria.

The coalition has been providing air support to the SDF to the tune of 150 strikes on ISIS positions since the assault began Tuesday, according to the Syrian Observatory for Human Rights.

"There has been a serious intensification of airstrikes, but they were most intense on the first day of the operation," said Observatory head Rami Abdel Rahman on Friday.

SDF forces have pushed forward from Ain Issa, which lies less than 60 kilometers north of Raqqa city, into the surrounding farmland and small villages made up of low buildings.

The fighting and bombardment has left 31 IS fighters dead so far, Abdel Rahman said, according to AFP.

The SDF has not released a tally of casualties among their forces, and the Britain-based Observatory could not immediately provide a toll.

"There are almost no civilians in the villages where the fighting is happening, which is why there is no civilian death toll," Abdel Rahman told AFP.