Story highlights Anti-terror police make arrests in Istanbul, Kocaeli, Mersin and Sanliurfa areas, news agency reports

Three foreigners who are among the 21 arrested were preparing to cross into Syria, Anadolu says

Istanbul (CNN) Turkish authorities have arrested 21 people -- including three foreigners -- suspected of belonging to ISIS, Turkey's semiofficial Anadolu news agency reported Friday.

Anti-terror police carried out simultaneous raids in four areas, including Istanbul, the news agency said. The other arrests were in northwestern Kocaeli province and the southern provinces of Sanliurfa, which borders Syria, and Mersin.

Those arrested are accused of being members of ISIS who helped recruit people to join the extremist group, Anadolu said.

The three foreigners were preparing to cross the border into Syria, it added.

Turkey has become a popular transit route for people seeking to reach Syria , where ISIS and other opposition groups have been battling the forces of Syrian President Bashar al-Assad

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