Islamic State has launched a major offensive along Syria's border with Turkey, seizing two villages captured by Western-backed rebels two days ago.

The Britain-based Syrian Observatory for Human Rights, which relies on activists inside the country, said IS militants seized Sheikh Reeh and al-Bal on Sunday.

The activist-run Azaz Media Centre, based in a northern town of the same name, said the extremists exploded seven bombs around Marea and other villages and towns, calling it the fiercest IS offensive in a year.

The IS-linked Aamaq News Agency claimed the capture of the two villages and said a suicide operation targeted rebel fighters in opposition-held Kafr Shousha.

The Western-backed Free Syrian Army captured the border town of al-Rai from IS on Friday.

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