Turkey has warned Syria's opposition in exile it could be the target of an attack in Istanbul by groups linked to al Qaeda or loyal to Syrian President Bashar al-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 Qaeda-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.
"We were given a piece of paper which we had to sign. It said there was a risk of attack from the (Syrian) regime ... and from ISIL," a senior National Coalition member told Reuters.
"They said from the regime it could be a car bomb and from ISIL it could be a suicide bomber," the official said.
Two other opposition sources confirmed the warning.
Reuters has not seen the document, which the opposition sources said was given to them several weeks ago.
A spokesman for Turkey's foreign ministry said he was not aware of a written warning but said Turkish security forces were in constant contact with National Coalition members.
Recent opposition meetings in Turkey have been held at secret locations, partly because of security concerns. Opposition representatives were attending peace talks with Damascus officials in Switzerland on Wednesday, but the prospects of any agreement appeared remote.