A suicide bombing Tuesday killed 10 people, including nine Germans, in the tourist heart of Istanbul, with officials saying the attacker was a man with links to Syria.
A source in the Turkish Prime Minister's Office said the Germans were among the dead in the Sultanahmet area. The state run Anadolu news agency reported that Prime Minister Ahmet Davutoglu has spoken with German Chancellor Angela Merkel.
"We are deeply concerned that German citizens could be and probably are among the victims and injured," Merkel had said earlier in Berlin.
Turkish Deputy Prime Minister Numan confirmed that foreigners were among the dead, but did not specify nationalities. He identified the suspect as a Syrian born in 1988.
Additionally, of the 15 people injured, two are in serious condition, the minister said.
The Norwegian Foreign Ministry confirmed one citizen sustained "minor injuries." CNN Turk reported a Peruvian national was also injured.
The German Foreign Office said it was in touch with the Turkish authorities and was urging citizens to avoid crowded areas in Istanbul.
The large blast, which could be heard several kilometres away, took place at around 10:15 am (0815 GMT) in Sultanahmet, home to the Hagia Sophia museum and the Blue Mosque, both major tourist attractions on the European side of the metropolis.
Speaking in Ankara, President Recep Tayyip Erdogan condemned the "terrorist" attack and said that a person of Syrian origin was the perpetrator, according to initial assessments.
No group has claimed responsibility for the attack, which comes amid rising violence in Turkey.
The authorities blamed three major suicide attacks last year on the Islamic State extremist group, which controls territory in neighbouring Syria and Iraq. The group has never claimed an attack in Turkey. There is also ongoing fighting with Kurdish militants.
Prime Minister Ahmet Davutoglu was to convene a security meeting in Ankara.
A large group of police and emergency workers were at the scene after police cordoned off the area near the blast.
The government has also imposed a temporary broadcast ban in the wake of the explosion.
State television and other major stations were not showing images from the scene. Reporters in Sultanahmet, including a dpa journalist, had difficulties photographing and accessing the site.
Turkey - which borders both Iraq and Syria, two nations in the throes of civil wars - has been facing increasing unrest during the past year.
The largest blast in the country's history took place in October, in Ankara. Two suicide bombers blew themselves up near a train station during a pro-Kurdish peace rally, killing 100 people.
Islamic State militants were blamed for that attack, as well as for another suicide blast in July in the south of the country, which left more than 30 people dead. The group never claimed responsibility.
Turkey stepped up its fight against Islamic State last year, after a period in which it was criticized for being slow to tackle the threat from the extremist group.
Meanwhile, violence in the south-east of the country has soared, as state security forces battle Kurdish militants after a peace process collapsed in the middle of last year.
Erdogan used his speech following the blast to also launch into a criticism of academics who have chastised Turkey's human rights policy in mostly Kurdish areas of the country.
Hundreds have died in the south-eastern Turkey fighting, including members of the security forces, militants from the Kurdistan Workers’ Party (PKK) and civilians. Strict curfews have been imposed on some districts, drawing concern from human rights groups.
Istanbul has seen sporadic violence too, mostly from far-left groups. A mortar explosion at an airport in the city last month left one dead, with no clear claim of responsibility.
The last major terrorist attack in the city took place in 2003, when suspected al-Qaeda-affiliated militants detonated four truck bombs in two days, killing at least 57 people. The attacks targeted Jewish synagogues, a bank and the British Consulate.