Turkish forensic police officers walk near Santa Maria church after an attack in Istanbul on Sunday. AFP

The attack occurred at around 11:40 am (0840 GMT) at the Santa Maria church in the Sariyer district of Istanbul on the European side of Turkey's largest city and was carried out by two masked men, Interior Minister Ali Yerlikaya said on social media.

Turkish officials said it looked like a targeted attack against one person rather than against the Catholic church.

The minister said an individual identified only as C.T. -- who was among those attending Sunday's service -- was the target of the gun attack and lost his life.

Yerlikaya added that an investigation had been launched to find the attackers, who fled the scene after the shooting.

Local officials said around 40 people attended the mass, and suggested that there could have been more casualties.

"After the second shot, the gun didn't work, then they (attackers) ran. At this moment, everyone lied on the floor. There were around 35 to 40 people inside," Sariyer district's mayor Sukru Genc told reporters.

Istanbul governor Davut Gul said that there were no other injuries.

"We strongly condemn this vile attack," Yerlikaya said.

Security footage ahead of the attack showed a pair of men wearing black ski masks with their hands hidden in their pockets and one was seen wearing black sunglasses.

President Recep Tayyip Erdogan, who was in the central Anatolian province of Eskisehir for his party's rally ahead of March local elections, expressed condolences during a phone call with the priest of the Italian church and other local officials.

He assured that "necessary steps are being taken to catch the perpatrators as soon as possible", according to his office.

Pope Francis expressed his support for the Catholic church after the attack.

"I express my closeness to the community of the Santa Maria" church in Istanbul, the Argentinian pope said at the end of his weekly Angelus prayer in St Peter's Square at the Vatican.