AFP

Warplanes from a U.S.-led coalition have conducted at least 150 air strikes on the Islamic State group to bolster a major offensive on the jihadists' Syrian stronghold of Raqa, a monitor said Friday.

The U.S. is backing twin assaults against IS -- one for Raqa 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 Raqa city -- IS's bolthole in Syria.

Turkey on Friday said it was "unacceptable" that U.S. troops had been seen near Raqa wearing insignia of the Kurdish People's Protection Units (YPG), which makes up the bulk of U.S.-backed Syrian Democratic Forces and which Ankara regards as a terror group.

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

"There has been a serious intensification of air strikes, 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 (40 miles) north of Raqa 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.

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.

- Raqa residents terrified -

Near the front line, an AFP photographer on Wednesday saw U.S. soldiers supporting SDF forces, who say they have advanced seven kilometers from Ain Issa.

"We liberated the villages of Fatisah, Namroudiya, and Wastah as well as several fields. The coming battle will hold a lot of big surprises," SDF field commander Baraa al-Ghanem told AFP.

Further south in Raqa city, the estimated 300,000 people still living there were becoming increasingly desperate to flee.

According to anti-IS activist group Raqa is Being Slaughtered Silently (RBSS), residents were paying smugglers $400 (350 euros) each to try to escape the city.

"There is nearly no one walking in the streets," said RBSS activist Hamoud al-Musa, who hails from Raqa.

"People are afraid of a brutal onslaught from the warplanes, whether coalition, Russian, or even regime," he told AFP.

IS had set up a few new checkpoints in Raqa city and was "amassing its forces on the front lines" further north, he said.

IS members were also doubling down on mandatory religious training for residents and "stressing the importance of jihad and calling people to jihad," Musa said.

The extremists have tightened restrictions on people leaving the city, amid accusations that they are using the civilian population as human shields.

Abdel Rahman said a handful of families had been able to escape the city and had made it west to Idlib province, controlled by a rebel alliance including IS rival, Al-Nusra Front.

- 'Harrowing' escape -

Across the border in Iraq, pro-government forces have advanced on bridges leading to IS-held Fallujah, said Staff Lieutenant General Abdulwahab al-Saadi, head of the Fallujah Liberation Operations Command.

But IS fighters were using "car bomb and suicide (bombers) and sniper detachments" to resist the advance.

About 50,000 civilians are estimated to be trapped inside the city, and only 800 had been able to escape thus far, according to the U.N.'s refugee agency.

Spokesman Melissa Fleming said the U.N. had received reports that people including women and children had been killed trying to flee.

"Inside Fallujah, there have been reports of a dramatic increase in the number of executions of men and older boys in Fallujah refusing to fight on behalf of extremist forces," Fleming said.

She described "harrowing tales" of families trekking for hours through the night on foot, sometimes hiding in old irrigation pipes, to reach safety.

Fallujah, which lies only 50 kilometers (30 miles) west of Baghdad, has been out of government control since January 2014 and is one of only two remaining major Iraqi cities still in IS hands, the other being Mosul.

Fleming said a total of 4,266 people had fled Mosul and traveled across the border to Syria's Hasakeh province.