An Islamic State suicide bomber has detonated a bomb in a historic district of Istanbul popular with tourists, killing 10 people - at least nine of them German - and wounding 15 others, Turkish officials said.
Prime minister Ahmet Davutoglu said IS was behind the suicide attack, adding that Turkey is determined to battle the militant group until it no longer "remains a threat" to the country or the world.
Turkey's state-run news agency said Mr Davutoglu held a telephone conversation with German chancellor Angela Merkel to express his condolences.
A senior government official confirmed that most of the victims were German. Ms Merkel had earlier said they were part of a German travel group.
"I strongly condemn the terror incident that occurred in Istanbul, at the Sultanahmet Square, and which has been assessed as being an attack by a Syria-rooted suicide bomber," president Recep Tayyip Erdogan said.
Deputy prime minister Numan Kurtulmus said the bomber was a 28-year-old Syrian national whose "connections" were being investigated. It was unclear whether the death toll of 10 included the bomber.
Ms Merkel, speaking at a news conference in Berlin, condemned the attack: "Today Istanbul was hit, Paris has been hit, Tunisia has been hit, Ankara has been hit before. International terrorism is once again showing its cruel and inhuman face today."
The explosion, which could be heard over a wide area, hit a park that is home to a landmark obelisk, 30 yards from the historic Blue Mosque.
Turkey's Dogan news agency said one Norwegian and a Peruvian were also among the wounded, and Seoul's Foreign Ministry said a South Korean had a finger injury. The Norwegian Foreign Ministry told Norway's news agency NTB that the Norwegian tourist was slightly hurt and was being treated in a local hospital.
Mr Kurtulmus said two of the wounded were in serious condition.
Germany and Denmark have warned their citizens to avoid crowds outside tourist attractions in Istanbul.
Last year, Turkey agreed to take a more active role in the US-led battle against IS. Turkey opened its bases to US aircraft to launch air raids on the extremist group in Syria and has carried out a limited number of strikes itself.
It has also moved to tighten security along its 560-mile border with Syria in a bid to stem the flow of militants.
Police sealed off the Sultanahmet area, barring people from approaching in case of a second explosion, and a police helicopter hovered overhead.
Sultanahmet is Istanbul's main sightseeing area and includes the Topkapi Palace and the former Byzantine church of Haghia Sophia, now a museum.
Mr Davutoglu immediately convened a security meeting with the country's interior minister and other officials.
As with previous attacks, authorities imposed a news blackout, barring media from showing images of the dead or injured or reporting any details of the investigation.
Turkey suffered two major bombing attacks last year, both blamed on Islamic State.
More than 30 people were killed in a suicide attack in the town of Suruc, near the border with Syria, in July.
Two suicide bombs exploded in October outside Ankara's main train station as people gathered for a peace rally, killing more than 100 in Turkey's deadliest attack. The prosecutor's office said that attack was carried out by a local IS cell.