Islamic State militants on Friday launched an attack in Syria's northern Aleppo province, seizing key territory from rebel groups near the Turkish border, activists and a monitoring group said.

The attack placed the city Marea under an effective siege and threatens the nearby city of Azaz along with the border crossing to Turkey - all of which are controlled by rebels opposed to both Islamic State and the Syrian government.

Aid groups, including Doctors Without Borders (MSF), estimate at least 100,000 people are now in the cross hairs of the new offensive, trapped between the front lines and the Turkish border, which has been largely closed to refugees for over a year.

Several villages have fallen to the jihadis, amid heavy fighting, according to the Syrian Observatory for Human Rights. One village, Kiljibreen, saw nine civilians killed in shelling.

"Islamic State is randomly shelling the areas of Kaljibreen, Kfar Kalbeen, Neyara and al-Nada, causing many casualties among the residents," said a local activist, who only gave his first name, Abu Gheith, citing safety concerns. 

He added in a Facebook message that some 15,000 civilians in Marea, which lies about 15 kilometres south of the border, are now under siege.

At least 30 rebels and 11 Islamic State fighters were killed in the fight, according to the Observatory, which relies on a network of activists inside Syria for its information.

The Amaaq news agency, which acts as a semi-official news agency for the hardline jihadis, confirmed that Islamic State seized a series of villages near Marea.

"People are in a state of panic inside Marea," said Abdel Rahman, an emergency worker with the White Helmets group, which operates in rebel held areas. "They do not know where to go."

Some people fleeing the violence in Marea have fled to the Kurdish Afrin enclave but others were reportedly wary of this route. There were later reports that the YPG had established checkpoints on the roads to its territory, upping security.

The northern parts of Aleppo were cut off in February from the south of the province, including the city of Aleppo, by a Russian-backed government offensive, with Iran also supporting President Bashar al-Assad's troops.

The offensive comes even as Islamic State is under pressure in north-eastern Syria, after Kurdish and allied Arab forces backed by the United States launched an attack this week on the northern side of al-Raqqa, edging closer to the militants' de facto capital.

US support for the Syrian Kurds has angered Turkey, where government forces have long fought Kurdish militants and are worried about rising nationalist sentiment and cross-border cooperation. Turkey is a major backer of rebels in northern Syria.

Turkish Foreign Minister Mevlut Cavusoglu slammed Washington after photos emerged of US soldiers in northern Syria wearing the insignia of the main Kurdish militia in the country as they jointly moved to attack Islamic State.

"It is unacceptable that US soldiers wear the emblem of a terrorist organization," Cavusoglu said in Antalya.

Pentagon spokesman Colonel Steven Warren said the soldiers had been ordered to remove the insignia.

"Wearing those YPG patches was unauthorized and it was inappropriate," Warren said. "Corrective action has been taken."

US special forces have a history of wearing such insignia when they are partnering with forces around the world, but it isn't authorized, Warren added.

He said the soldiers are part of a contingent of more than 200 US "advisers" in Syria working with the Syrian-Arab coalition in the fight against Islamic State.

Turkey considers the Kurdish People's Protection Units (YPG) militia a terrorist group, but its fighters are the main and most effective ally of the US pushing back Islamic State on the ground, having seized large swaths of north-eastern Syrian from the jihadis.

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