Anadolu Agency said Friday the suspects include the group's so-called "emir," or ruler, of Deir el-Zour, a major city in eastern Syria, and its environs. It described the other three suspects as senior operatives within the extremist group.
Hurriyet newspaper said the four were captured as part of a joint operation by Turkey's intelligence agency and police anti-terrorism units.
The suspects were hiding among a group of Syrian refugees planning to cross into Europe, the newspaper reported.
The four are being questioned by anti-terrorism police in Izmir.
Police in Izmir confirmed the operation but could not immediately provide details.
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