Thousands of civilians and hundreds of aid workers were "in immediate danger" in northern Syria, an aid group warned on Saturday, after a massive onslaught by Islamic State militants split the last rebel-held enclave north of Aleppo in two.

The "entire area of A'zaz," near the Turkish border, was unsafe and civilians need a humanitarian corridor to be allowed to get to safety, Karl Schembri, spokesman for the Norwegian Refugee Council in the Middle East, told dpa.

The jihadists stormed the defences of Marea, the main town in the southern part of the enclave, on Saturday after cutting it off from A'zaz the previous day, a monitoring group said.

Heavy battles were taking place in outer parts of the town, Rami Abdel-Rahman, head of the Syrian Observatory for Human Rights, said.

Ammar al-Najjar, an emergency rescue worker with the Syrian Civil Defence based in Marea, told dpa that 1,500-2,000 families were trying to leave the town.

In a sign of the rebels' weak position, they handed over the village of Sheikh Issa adjoining Marea to rival Kurdish forces to facilitate safe passage for those fleeing, an activist based on the Syrian-Turkish border, Ammar Jello, said. Abdel-Rahman of the Observatory confirmed the handover.

The fighting in the enclave leaves over 160,000 people who had taken refuge in the A'zaz area caught once more in a conflict zone, according to UN figures.

The threatened enclave is the last rebel-held territory north of Aleppo. In February it was cut off from Aleppo city by a Russian-backed government offensive.

It comes as the Kurdish-led Democratic Forces of Syria, backed by US special forces and airstrikes, mount an offensive further east on the hinterland of Islamic State's de facto Syrian capital of al-Raqqa.

Ankara, which distrusts the Kurds due to their links with Kurdish rebels operating inside Turkey, had hoped that the rebels would be able to capture areas east of A'zaz, which are the jihadist group's last remaining territories along the Turkish border.

The Kurds also have their eye on this territory, which would allow them to link up their main stronghold in northern Syria with the Efrin area north-west of Aleppo.

With Turkey objecting to any Kurdish move towards the border areas, the US apparently persuaded the Democratic Forces of Syria to push towards al-Raqqa instead.

At least 41 rebel fighters and 27 civilians have been killed in the fighting around A'zaz and Marea since Friday morning, and the jihadists have deployed nine suicide car bombs to break through rebel defences, the observatory said.

Nearby areas in Turkey have again come under rocket fire since Friday, apparently from Islamic State-held areas in Syria.

The United Nations Refugee Agency (UNHCR) said Saturday it is deeply concerned about the plight of some 165,000 displaced persons reportedly massing near A'zaz.

"People have started to flee due to heavy fighting in northern Aleppo. Fleeing civilians are being caught in crossfire and are facing challenges to access medical services, food, water and safety," it said in a statement.

Schembri said many of the displaced people in the area have already been forced out of their homes in al-Raqqa, Deir al-Zour in eastern Syria, and even Iraq.

"They urgently need a humanitarian corridor to be allowed to safety; their lives are in immediate danger," he warned.

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