Turkish special forces raided buildings in Istanbul used by suspected members of an Islamist militant group active in neighboring Syria and Iraq late on Monday, leaving three policemen and two suspects wounded, police said.
The five - including a man and a woman thought to belong to the Islamic State of Iraq and the Levant (ISIL) - were hit after people in the buildings opened fire on security forces, police added.
ISIL is among fragmented Islamist groups fighting against Syria's President Bashar al-Assad in a three-year conflict. It is has also battled Iraq's government in that country's western Anbar province.
Ankara has repeatedly denied it is arming rebels inside Syria, but its highly publicized opposition to Assad has raised fears that Turkey may have become a safe haven for Islamist militants battling against Damascus.
If ISIL's involvement is confirmed, it would be the first clash with the group inside a Turkish city.
The raid in Istanbul's residential Umraniye neighborhood came a week after two members of the security forces were killed in the southern Turkish province of Nigde when suspected members of ISIL opened fire from a truck, and just days before Turkey holds high stakes municipal polls.
Hundreds of thousands of refugees have fled into Turkey along its 900 km frontier with Syria during the brutal three year conflict, and cross border fire has occasionally led to civilian deaths in Turkish territory.
Turkish armed forces shot down a Syrian plane on Sunday that Ankara said had violated its air space.