"Today it hit Istanbul, before that it was Paris, Tunisia and Ankara," Chancellor Angela Merkel said after it emerged that nine out of 10 people killed in a terrorist attack in the Turkish capital on Tuesday were German nationals.
"International terrorism has once again reared its ugly head and this shows the necessity of proceeding with determination against it," the German leader said.
Merkel's comments seemed to confirm that the Islamic State extremist group was the prime suspect in the attacks, which occurred near the Hagia Sophia and the Blue Mosque - two famed tourist attractions in Istanbul's Sultanahmet district.
Of the 15 people that sustained injuries in the explosion, another nine were Germans thought to be part of a tourist group, Turkish officials said.
Turkish Prime Minister Ahmet Davutoglu spoke to Merkel on the phone to inform her about the incident and assure her that Turkey would share the details of the investigation with German officials. He also expressed his condolences about the deaths of German citizens.
He placed the blame for the attacks squarely with Islamic State in a nationally televised address.
German Foreign Minister Frank-Walter Steinmeier condemned the "barbaric, cowardly act of terror," and expressed his condolences to the victims' families.
The crisis committee of the German government was working with the Consulate General in Istanbul and the German embassy in Ankara to get to the bottom of the attack, Steinmeier said in a statement posted on Twitter.
His Foreign Ministry advised German citizens travelling in Turkey to avoid large congregations of people. "Political tensions, violent confrontations and terrorist attacks can be expected across the country," the ministry said on its website.
German tourism companies were offering customers with bookings for holidays in Turkey free cancellations. Travel company Alltours said that it had removed Istanbul from its Turkey itineraries several weeks ago due to the continued security risk.
German visitors are vital to Turkey's tourism industry. In the first 10 months of 2015, the largest group of foreigners visiting Turkey were Germans - over 5 million people, constituting 15 per cent of the total number of visitors.