One person was shot dead, according to Interior Minister Ali Yerlikaya.

He added that authorities were working to capture the assailants.

Yerlikaya said the attack, which Ankara condemned, took place around 0840 GMT at the Italian Santa Maria Catholic Church in the Sariyer district.

And that one person - who was targeted by the gunmen - was killed while attending the service.

Speaking to reporters outside the church, Istanbul's governor said the victim was a Turkish national and that no one else was hurt.

He said the attackers only fired at one person.

It was not immediately clear what the motive for the attack was or why the victim was targeted.

President Tayyip Erdogan said the perpetrators would be found "very soon."

Pope Francis conveyed his condolences over the attack after his weekly Angelus prayer.