A suicide bomber with links to Syria is thought to have been responsible for killing at least ten people in central Istanbul, the country's president has announced.
The death toll may yet climb, with at least 15 people injured as the blast tore through the historic Sultanahmet Square, an area popular with tourists, early on Tuesday morning.
"Very concerned at reports of explosion in Istanbul," tweeted Richard Moore, the UK ambassador to Turkey. "In touch with Turkish authorities & thoughts with those injured. Nationalities of victims currently unknown."
At least one of the injured is thought to have been from South Korea, an official at Seoul's consulate has said.
Germans, Norwegians and a Peruvian were also thought to be among the casualties, though it remains unclear whether they are among the injured or the deceased.
"We mourn the victims of this murderous attack," tweeted German foreign minister Frank-Walter Steinmeier. "We can't exclude that Germans are among the dead."
Police have cordoned off the area near the famed Blue Mosque and Hagia Sophea to begin initial investigations, but President Recep Tayyip Erdogan has announced that the suspected bomber "was of Syrian origin" after holding an emergency meeting with advisers and senior officials.
The suspected bomber was not, however, on Turkey's watch-list of suspected Islamic State group militants, said Numan Kurtulmuş, Turkey's deputy prime minister.
BBC World News reporter Seref Isler has tweeted that there is "a media blackout" in the country over the Istanbul explosion.
Our own correspondent in Istanbul has confirmed that local media has been banned from publishing details of the explosion.
Elsewhere, in what is thought to be an unrelated incident, a large fire broke out at a 12-storey hotel in the city's Maltepe district, with several people feared trapped inside. Authorities later said the blaze had been contained, and no injuries were reported.
In a third disturbance, a fire has erupted at the security directorate in Turkey's capital, Ankara. Authorities there are investigating, but it is also believed at this time to be unrelated to the other events rocking the country on Tuesday.