The blast went off at around 10 a.m. in a park outside the historic Blue Mosque, leaving bodies strewn on the ground.

Turkish President Recep Tayyip Erdogan said at a speech that the suspected suicide bomber was of "Syrian origin" without specifying whether the bombing was carried out by the "Islamic State."

Istanbul's governor said 10 people were killed and at least 15 wounded. It is unclear whether the dead were Turkish citizens or foreigners.

Six Germans as well as Norwegian, South Korean and Peruvian citizens were among those being treated for wounds at a local hospitals, CNN Turk reported.

Witnesses described to CNN Turk a loud noise and fire-like explosion.

Police sealed off the area and shut down transportation as ambulances responded to the scene.

The German Foreign Office warned its citizens to avoid crowds and tourist sites, saying that violent clashes and terror attacks were expected across the country.

Chancellor Angela Merkel said that it was possible German citizens were among the dead and wounded.

Citing national security, the government issued a temporary news ban on the blast, forcing news organizations to stop coverage.

Kurdish, far-left and Islamist militants have carried out bomb attacks in Turkey in the past. The so-called "Islamic State" carried out two suicide bombings last year in Turkey, killing more than 130 people.

The Blue Mosque, Aya Sofya and Topkapi Palace draw millions of tourists a year to the Sultanahmet district.