Warplanes from the US-led coalition have pounded the Islamic State group with at least 150 strikes to bolster a major offensive on the jihadists' Syrian stronghold of Raqa, a monitor said Friday.
The US is backing twin assaults against IS -- one in Raqa province and another which aims to retake the Iraqi city of Fallujah across the border.
A Kurdish-Arab alliance is being supported by coalition air raids as well as US forces on the ground in its push for territory north of Raqa city -- IS's de facto Syrian capital.
Turkey on Friday said it was "unacceptable" that US troops had been seen near Raqa wearing insignia of Kurdish militia who belong to the US-backed Syrian Democratic Forces (SDF) and who Ankara regards as a terror group.
The coalition has been providing air support to the SDF with 150 strikes on IS positions since the assault began Tuesday, according to the Syrian Observatory for Human Rights, a British-based monitor.
"There has been a serious intensification of air strikes," Observatory head Rami Abdel Rahman said.
SDF forces have pushed forward from Ain Issa, less than 60 kilometres (40 miles) north of Raqa city, into the surrounding farmland and small villages.
The fighting and bombardment has left 31 IS fighters dead so far, Abdel Rahman said.
The number of SDF casualties was unclear.
Near the front line, an AFP photographer on Wednesday saw US soldiers supporting SDF forces, who say they have advanced seven kilometres from Ain Issa.
The twin offensives come as world powers try to salvage a shaky ceasefire between the regime and non-jihadist rebels agreed in February to boost efforts to end a conflict that has killed more than 280,000 people.
- Raqa residents terrified -
The estimated 300,000 people still living in Raqa city are 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.
"There is nearly no one walking in the streets," said RBSS activist Hamoud al-Musa.
"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.
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For the second time this week, coalition warplanes on Friday morning dropped leaflets encouraging residents to flee Raqa.
IS, which has tightened restrictions on movement, has been accused of using residents as human shields.
Abdel Rahman said a handful of families had fled the city to Idlib province, controlled by a rebel alliance including IS's jihadist rival, Al-Nusra Front.
- IS shock advance -
IS swept through rebel territory in Aleppo province Friday in a shock advance, cutting off tens of thousands of internally displaced Syrians living in informal camps near the closed Turkish border.
Pablo Marco, regional operations manager for Doctors Without Borders (MSF), said the group was "terribly concerned... about the estimated 100,000 people trapped between the Turkish border and active front lines."
In Aleppo city, at least four civilians including a child were killed in barrel bomb attacks on an opposition-controlled eastern district, according to the civil defence.
Air strikes also killed 11 people in a bakery in the town of Hreitan and four in Kfar Hamra in the same province, rescue workers said.
Rebel rocket fire hit Aleppo's regime-held district of Midan, killing an elderly woman and wounding nine others, state media said.
In Iraq, pro-government forces have advanced towards bridges leading to IS-held Fallujah, said Staff Lieutenant General Abdulwahab al-Saadi, head of the Fallujah Liberation Operations Command.
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, according to the UN's refugee agency.
Spokeswoman Melissa Fleming said the UN had received reports that people including women and children had been killed trying to flee.
"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 kilometres (30 miles) west of Baghdad, has been out of government control since January 2014.
The operation to retake what is one of only two remaining major Iraqi cities still in IS hands has been complicated by a political crisis in the capital that saw security forces fire tear gas Friday as thousands of protesters massed.
Iraqi Prime Minister Haider al-Abadi had urged the demonstrators to stay at home because security forces are busy trying to recapture Fallujah.