Thousands flee IS offensive in northern Syria: Monitor

Thousands of civilians have fled an offensive by the Islamic State (IS) group against rebels in northern Syria into territory controlled by a US-backed Kurdish-led alliance, a monitor said on Sunday.

The offensive against the towns of Marea and Azaz threatens to overrun the last swath of territory in the east of Aleppo province held by rebels and bring IS to the doorstep of the Kurds' Afrin enclave.

At least 29 civilians have been killed since IS launched the assault early on Friday, the Syrian Observatory for Human Rights said.

It came as IS were under attack by the Kurdish-led alliance in Raqqa province further east and by the army and allied militia around Fallujah in neighbouring Iraq. 

"More than 6,000 civilians, most of them women and children, were able to flee areas in the countryside of Aleppo province ... especially from Marea town and Sheikh Issa village" to its west, the Britain-based monitoring group said.

"The displaced arrived last night in areas in the west and north of Aleppo province under the control of the Syrian Democratic Forces," Observatory chief Rami Abdel Rahman said.

The SDF is an alliance of Kurdish and Arab fighters that Washington regards as the most effective force on the ground in Syria against IS.

Washington's support for the alliance, which is dominated by the Kurdish People's Protection Units (YPG), has severely strained relations with NATO ally Ankara, which regards it as a terror group.

AFP pictures of US commandos wearing the YPG insignia drew condemnation on Saturday from Turkish President Recep Tayyip Erdogan, whose government regards the group as a puppet of the Kurdistan Workers' Party (PKK), which Ankara has been battling for more than three decades.

IS launched its offensive against the rebel-held territory that separates it from the Kurds in Afrin on Friday.

Heavy fighting raged early on Sunday on the outskirts of Marea and around two villages on the supply route to the town of Azaz on the Turkish border to the northeast.

IS managed to cut the key supply line in a surprise assault early on Friday.

At least 61 rebel fighters have been killed in the fighting, as well as 47 IS militants, nine of them suicide bombers, the Observatory said.

The United Nations has expressed concern for some 165,000 civilians who have been trapped by the fighting between Azaz and the closed Turkish border.

The UN refugee agency said fleeing civilians were being caught in crossfire and were facing "challenges to access medical services, food, water and safety". 

The supply lines to Turkey have made Aleppo province one of the most contested battlegrounds of Syria's five-year-old civil war.

Parts are held by the government, parts by non-jihadist rebels, parts by the Kurds and parts by IS or its rival al-Qaeda.