Turkish Prime Minister Ahmet Davutoglu has said a suicide bombing in central Istanbul that left 10 people dead, most of them Germans, was carried out by a member of the Islamic State (IS) militant group.
All of those killed in the January 12 incident were foreigners, he said.
"We have determined that the perpetrator of the attack is a foreigner who is a member of Daesh," Davutoglu told reporters in the capital, Ankara, using an Arabic acronym for IS.
Fifteen more people, also mostly foreigners, were injured in the blast in the Sultanahmet district, just meters from Istanbul's historic Blue Mosque.
"I strongly condemn the terror attack, which was carried out by a suicide bomber of Syrian origin," Turkish President Recep Tayyip Erdogan said in televised remarks in the capital, Ankara.
"The first target of all the terror groups active in this region is Turkey, because Turkey fights them all with the same determination," Erdogan said.
LIVE BLOG: Read more about the Istanbul attack in Tracking Islamic State
Deputy Prime Minister Numan Kurtulmus said the bomber was a 28-year-old Syrian man.
Turkey's state media reported that Davutoglu had informed German Chancellor Angela Merkel in a telephone conversation about the German casualties.
Merkel acknowledged that it was feared that many Germans were among the casualties.
"We are seriously concerned that German citizens could be and probably are among the victims and injured," Merkel said in Berlin.
"Today Istanbul was hit. Paris has been hit, Tunisia has been hit, Ankara has been hit before," Merkel said. "International terrorism is once again showing its cruel and inhuman face today."
The White House condemned the "heinous attack" and pledged solidarity with NATO ally Turkey against terrorism. UN Secretary-General Ban Ki-moon said he hoped those responsible for "this despicable crime" were swiftly brought to justice.
Television footage showed several bodies and body parts lying on the ground. Ambulances rushed to the scene of the blast, while police cordoned off the area, taking precautions against a possible second explosion.
PHOTO GALLERY: Images From Scene Of The Blast
Central Istanbul Blast Kills At least 10
Davutoglu's office imposed a broadcast ban on reporting of the attack, prompting television channels to halt live broadcasting from the scene. The move prompted criticism.
Davutoglu also held an emergency security meeting of key ministers and officials. Among those taking part were in the meeting were Interior Minister Efkan Ala and intelligence chief Hakan Fidan, state-run Anatolia news agency reported.
Norway's Foreign Ministry said one Norwegian man was injured in the attack, while Germany warned its citizens to avoid crowds and tourist sites in Istanbul.
"Travelers in Istanbul are strongly urged to avoid for now large groups of people in public places as well as tourist attractions and to stay abreast of the situation via these official travel advisories and the media," the German Foreign Ministry said in a statement.
The European Union said after the blast that it stood with Turkey in the fight "against all forms of terrorism."
Turkey and the EU "must step up our efforts to counter extremist violence," EU foreign-policy chief Federica Mogherini said in a statement.
Kurdish, leftist, and Islamist militants have all carried out attacks in Turkey in recent months.
The January 12 blast comes just over a year after a female suicide bomber blew herself up in the same area, killing one police officer and wounding another.
A far-left group initially claimed that attack, but officials later said a woman with suspected Islamist militant links was responsible.
Turkey also remains on high alert after a series of attacks that the authorities said were perpetrated by IS.
In October, two suicide bombings in Ankara killed 103 people. Prosecutors said the attacks were carried out by IS militants.
Violence has also escalated in the mainly Kurdish southeast since a two-year cease-fire collapsed in July between the Turkish government and Kurdistan Workers Party (PKK) militants, after a suicide bombing by suspected IS militants killed at least 30 people in the town of Suruc on the Syrian border.
That attack targeted a cultural center as a Kurdish political group was conducting a press conference.
The PKK, which has been fighting for three decades for Kurdish autonomy, accused the Turkish security forces of collaborating with IS.
However, the Kurdish militant group has generally refrained from attacking civilian targets in urban centers outside the southeast in recent years.