Turkey's state-run news agency says police carried out raids in Istanbul and three other cities, detaining at least 21 people suspected of being members of the Islamic State extremist group.

The Anadolu Agency said three of the suspects detained in the simultaneous raids on Friday were foreign nationals who were planning to cross into Syria to join IS. Their nationalities weren't disclosed.

Other suspects were believed to have been working to recruit fighters for the group, the agency said.

Anadolu said police seized two rifles, large amounts of ammunition, documents and military uniforms during the raids.

Under pressure from its allies, Turkey has recently tightened security along its border with Syria in a bid to thwart militants from crossing into the country to join the extremists.