Freelance journalist Zia Weise has posted this photo taken from a rooftop looking out toward Istanbul's Blue Mosque.
Turkish Interior Minister Efkan Ala has arrived at the Prime Minister's residence in order to brief Prime Minister Ahmet Davutoğlu on the attack in Sultanahmet, the state-run Anadolu Agency has reported.
German and Norwegian tourists are among those killed in the blast this morning at Istanbul's main tourist district, Euronews is reporting, saying that the victims were part of a tour group.
CNN Turk is still reporting on the explosion in Istanbul despite a media blackout.
According to CNN Turk, police are focussing on the possibility that a suicide bomber was responsible for the blast.
Another photo from Istanbul's Sultanahmet district, the site of this morning's explosion.
The explosion in Istanbul occurred at about 10:20 a.m. local time this morning in Sultanahmet Square in Istanbul near the Obelisk of Theodosius, Hurriyet reports.
The explosion was heard in many of Istanbul's districts.
So far, reports say that ten people have been killed and 15 more injured.
The U.S. Embassy in Turkey has tweeted to say that it is closely following the reports of the explosion in Istanbul.
It is not known who was behind the blast in Istanbul -- though there is plenty of speculation on social media, with some people saying that the IS group may be responsible.
The Istanbul governor's office said in a statement that "Investigations into the cause of the explosion, the type of explosion and perpetrator or perpetrators are underway."
Now Turkey's Hurriyet News is also reporting that a gag order has been imposed over reporting on this morning's blast in Istanbul, so that confirms the rumors.
There are a number of reports emerging via social media of a gag order being imposed over the blast in Istanbul, but these are not confirmed.