ISTANBUL - Turkey has warned Syria's opposition in exile it could be the target of an attack in Istanbul by groups linked to al-Qaida or loyal to Syrian President Bashar Assad, opposition sources told Reuters.

Turkey has been a strong supporter of the Syrian opposition, hosting the moderate National Coalition in Istanbul and maintaining an open border policy, providing a life line to rebel-held areas by letting aid in and refugees out.

But the rise of al-Qaida-linked groups such as Jabhat al-Nusra and the Islamic State of Iraq and the Levant (ISIL) in parts of northern Syria near the border poses a growing threat.

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