Police staged raids across Turkey on Friday and detained 21 people, most suspected of belonging to Islamic State, the state-run Anadolu news agency reported.
Three of the detainees were foreigners, and authorities believe they were planning to enter Syria to fight for Islamic State, which is battling both President Bashar al-Assad's forces and opposition groups backed by Turkey's Western allies.
Under pressure from Nato members, Turkey has stepped up efforts to prevent foreign fighters crossing into neighbouring Syria to join Islamic State, which espouses a fundamentalist Islamic ideology.
Prime Minister Ahmet Davutoglu has said the Syrian conflict poses a major threat to Turkey. Thousands of foreigners have been barred from entering because of security concerns.
Officials at the counter-terrorism department in Istanbul's police headquarters declined to comment.
Authorities seized hunting rifles and ammunition in the simultaneous dawn operations in Istanbul and the nearby town of Kocaeli, as well as in Sanliurfa and Mersin, which are in southern Turkey near the Syrian border, Anadolu said.