The extremist group said, according to The Associated Press, that it "attacked a gathering of Christian unbelievers during their polytheistic ceremony" inside Santa Maria Church in the Buyukdere neighborhood in Istanbul Jan. 28.
Pope Francis has expressed his closeness to the small Catholic community in Turkey.
The shooting occurred just before noon at the church in the Sariyer district of Istanbul and was reportedly carried out by two masked men, according to eyewitnesses. Turkey's Interior Minister Ali Yerlikaya posted a statement on X, formerly known as Twitter, saying, "We strongly condemn this vile attack."
The area where the attack happened is popular with Christians in the 99.8%-Muslim majority country of nearly 85 million people, being home to Santa Maria Catholic Church -- known as the Italian church -- as well as a Greek Orthodox church and an Armenian Apostolic church, all of which date from the 19th century. Istanbul also is home to Patriarch Bartholomew I of Constantinople, spiritual leader of the world's Eastern Orthodox Christians.
Justice Minister Yilmaz Tunc said in a statement that "efforts continue to identify and capture the suspects who carried out the attack."
"The investigation is being carried out in a multifaceted and meticulous manner," Tunc added.
Television images from state media showed police and an ambulance outside the church, which is in an area of Istanbul popular with the expatriate and diplomatic community.
Turkey's ruling AKP party spokesman Omer Celik said on X that the attackers took aim at a person during the Mass.
"Our security forces are conducting a large-scale investigation into the matter," he wrote.
"Those who threaten the peace and security of our citizens will never achieve their goals," he insisted.
Local media reported that others were injured in the attack, but this was not immediately confirmed by the authorities.
Speaking at the Vatican during his traditional Sunday Angelus address, Pope Francis expressed his support as details of the incident emerged.
"I would like to express my closeness to the community of the church of Santa Maria in Istanbul, which suffered an armed attack during Mass that caused one death and left several injured," he told thousands of pilgrims and visitors gathered in St. Peter's Square for his blessing.
Turkish security services have been engaged in operations against the Islamic State group in recent months.
In December, security forces detained 32 suspects over alleged links with an extremist Islamic militant organization which planned attacks on churches and synagogues, as well as the Iraqi embassy.
The group have carried out a string of attacks on Turkish soil, including against a nightclub in Istanbul in 2017 that left 39 people dead.