At least 18 Islamic State (IS) terrorists were killed in Syria in an operation carried out by Turkish military and the US-led coalition, an official said on Thursday.About 14 IS targets were destroyed in the operation conducted on Wednesday, which was carried out before the militants were preparing to stage attacks on Turkey, Xinhua news agency reported.Two armed vehicles, one gun emplacement and one defence building belonging to the terrorist group were also destroyed in the shelling.