Iraq's elite forces deployed around Fallujah on Saturday, marking a new phase in efforts to retake the jihadist bastion, as concern grew for trapped civilians there and in neighbouring Syria.

After almost a week of shaping operations around Fallujah, the arrival of the counter-terrorism service (CTS) signalled that an assault on the Islamic State group inside the city may be imminent.

The deployment of Iraq's best-trained and most battle-tested unit came as US-backed forces pressed simultaneous offensives against IS in both Iraq and Syria.

Abdelwahab al-Saadi, the top commander in charge of the Fallujah operation, said the CTS, police and tribal fighters had reached two camps south and east of the city.

"These forces will break into Fallujah in the next few hours to liberate it from Daesh," he said, using an acronym for IS.

Fallujah, 50 kilometres (30 miles) west of Baghdad, is one of the two remaining major Iraqi cities still in IS hands.

IS also advanced in Syria's northern Aleppo province and further east as a Kurdish-Arab alliance backed by the US-led coalition pressed an offensive against Raqa, the jihadists' de facto capital in Syria.

Raqa is home to an estimated 300,000 people and residents have been paying smugglers $400 (350 euros) each to try to flee after IS tightened restrictions on people leaving, the activist group Raqa is Being Slaughtered Silently has said.

Around 165,000 displaced Syrians are also trapped between the closed Turkish border and the town of Azaz, sparking UN concern.

- Civilians trapped -

"Fleeing civilians are being caught in crossfire and are facing challenges to access medical services, food, water and safety," the UN refugee agency said.

Concern is also mounting for an estimated 50,000 civilians thought to be trapped inside Fallujah.

"We are receiving hundreds of displaced Iraqis from the outskirts of Fallujah who are totally exhausted, afraid and hungry," said a statement from Nasr Muflahi, country director for the Norwegian Refugee Council.

"Thousands more remain trapped in the centre of Fallujah, cut off from aid and any form of protection."

The estimated 1,000 jihadists still in Fallujah are suspected of using civilians as human shields, but the UN refugee agency also said Iraqi forces had blocked supply routes, preventing people from leaving.

Fallujah fell out of government control even before IS swept through Iraq's Sunni Arab heartland in June 2014, and is one of IS's most iconic strongholds.

Tens of thousands of Iraqi forces, including the Hashed al-Shaabi umbrella group dominated by Tehran-backed Shiite militias, began a huge operation on May 22-23 to retake it.

Hashed al-Shaabi forces ("Population Mobilisation" in Arabic), the army and police have so far focused on areas east of Fallujah, without entering the city itself.

CTS spokesman Sabah al-Noman said the force plans to "break into the city", and the operation was now "shifting to urban warfare after Iraqi forces completed the siege of the city".

He would not comment on the timing of an assault.

Iraqi forces evacuated 460 people -- mostly women and children -- on Friday, a senior police officer has said.

Fallujah and Mosul are the two major Iraqi cities still controlled by IS.

- Town encircled -

In Syria, heavy fighting between IS and rebels gripped the outskirts of the opposition-held town of Marea on Saturday, a monitor and activist said.

IS attacked mainly from the east and north using tanks and two car bombs, said Maamoun Khateeb, a journalist and activist from Azaz.

"The situation of the displaced in the Azaz area is really bad" and will not improve as long as Turkey keeps its border closed to refugees, he said.

The UN Office for the Coordination of Humanitarian Affairs said the fighting had encircled some 13,500 people in the town.

IS swept through rebel-held territory in Aleppo province early Friday in a shock offensive, cutting off the main road between Marea and Azaz.

The two towns have been vital stops along a rebel supply route from Turkey since 2012.

Further east, US-led coalition warplanes targeted IS positions north of Raqa, killing 45 IS fighters, the Syrian Observatory for Human Rights said.

But the Syrian Democratic Forces, the Kurdish-Arab alliance fighting IS, was struggling to make further progress five days into its offensive, it said.

Turkey's president meanwhile condemned US support for the alliance after AFP pictures revealed US commandos wearing the insignia of its Kurdish units, which Ankara has branded a terror group.

"Those who are our friends, who are with us in NATO... cannot, must not send their soldiers to Syria wearing YPG insignia," Recep Tayyip Erdogan said.

"The PKK, the PYD, the YPG, Daesh, there is no difference. They are all terrorists," Erdogan said of Kurdish groups and IS.

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