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IS militants fight Syria rebels in north Syria strongholds

  • FILE - In this Tuesday, Oct. 2, 2012 file photo, smoke rises over Saif Al Dawla district, in Aleppo, Syria. Islamic State militants entered a Syrian opposition stronghold in the country's north on Saturday, clashing with rebels on the edges of the town as the extremist group built on its most significant advance near the Turkish border in two years, Syrian opposition groups and IS media said. (AP Photo/ Manu Brabo, File)

    FILE - In this Tuesday, Oct. 2, 2012 file photo, smoke rises over Saif Al Dawla district, in Aleppo, Syria. Islamic State militants entered a Syrian opposition stronghold in the country's north on Saturday, clashing with rebels on the edges of the town as the extremist group built on its most significant advance near the Turkish border in two years, Syrian opposition groups and IS media said. (AP Photo/ Manu Brabo, File)  (The Associated Press)

  • FILE - In this Thursday, Feb. 11, 2016 file photo, civilians walk through a street with large tarps are hung in between building to limit the view of regime snipers in Aleppo, Syria. Islamic State militants entered a Syrian opposition stronghold in the country's north on Saturday, clashing with rebels on the edges of the town as the extremist group built on its most significant advance near the Turkish border in two years, Syrian opposition groups and IS media said. (Alexander Kots/Komsomolskaya Pravda via AP, File)

    FILE - In this Thursday, Feb. 11, 2016 file photo, civilians walk through a street with large tarps are hung in between building to limit the view of regime snipers in Aleppo, Syria. Islamic State militants entered a Syrian opposition stronghold in the country's north on Saturday, clashing with rebels on the edges of the town as the extremist group built on its most significant advance near the Turkish border in two years, Syrian opposition groups and IS media said. (Alexander Kots/Komsomolskaya Pravda via AP, File)  (The Associated Press)

  • FILE - In this Monday, Feb. 15, 2016 file photo, This image taken from video provided by the Syrian activist-based media group Azaz Media Center, which has-been verified and is consistent with other AP reporting, shows people gathered around destroyed vehicles in Azaz, Syria Syria. Islamic State militants entered a Syrian opposition stronghold in the country's north on Saturday, clashing with rebels on the edges of the town as the extremist group built on its most significant advance near the Turkish border in two years, Syrian opposition groups and IS media said. (Azaz Media Center,via AP Video, File) MANDATORY CREDIT

    FILE - In this Monday, Feb. 15, 2016 file photo, This image taken from video provided by the Syrian activist-based media group Azaz Media Center, which has-been verified and is consistent with other AP reporting, shows people gathered around destroyed vehicles in Azaz, Syria Syria. Islamic State militants entered a Syrian opposition stronghold in the country's north on Saturday, clashing with rebels on the edges of the town as the extremist group built on its most significant advance near the Turkish border in two years, Syrian opposition groups and IS media said. (Azaz Media Center,via AP Video, File) MANDATORY CREDIT  (The Associated Press)

Islamic State group militants have entered a Syrian opposition stronghold in the country's north and are clashing with rebels on the edges of the town.

Syrian opposition groups and IS report that the extremists advanced to the outskirts of Marea Saturday.

The advance — the biggest by IS in Aleppo province in two years — is part of an IS offensive targeting rebel strongholds near the Turkish border that began Thursday night.

On Friday, militants of the group seized six villages in advances that forced the evacuation of a hospital and trapped around 160,000 civilians amid heavy fighting. The advances brought the militants to within three 3 kilometers (2 miles) of the rebel-held town of Azaz and cut off supplies to Marea further south.