Federica Mogherini, the EU's foreign affairs chief, tweets:

The AFP news agency reports that a violent explosion has been heard in Ankara, while a US source tells Reuters that gunfire has been near the headquarters of the Turkish military's general staff in the capital.

The BBC's Katy Watson in Istanbul says this has come as a great surprise to Turks. 

People are glued to their phones, to their televisions, trying to understand what is going on, she adds.

The UK Foreign & Commonwealth Office tweets:

Turkish state broadcaster TRT has gone off air in the last few minutes, Reuters reports. Earlier, soldiers reportedly took control of its offices and an announcer read out a statement declaring martial law.

A European Union source tells the Reuters news agency: "It looks like a relatively well orchestrated coup by a substantial body of the military, not just a few colonels." "They control several strategic points in Istanbul. Given the scale of the operation, it is difficult to imagine they will stop short of prevailing," the source adds.

Eliot Higgins, of the Bellingcat website for citizen investigative journalists, is in Turkey and just posted this:

Turkey's Prime Minister Binali Yildirim has meanwhile been posting a lot on Twitter. In one post, he says the country will "never, ever tolerate unlawfulness"  and not allow democracy to be interfered with.

In his first comments today, President Recep Tayyip Erdogan tells CNN Turk, in a webcam interview, that this is an act encouraged by a "parallel structure". He says the coup attempt will be given the "necessary response" and calls on Turkish people to go out on the streets.

President Recep Tayyip Erdogan and his government are still in power and will not tolerate attempts to undermine democracy, a presidential source tells the Reuters news agency.

The US state department issues a travel alert telling US citizens in Turkey "shelter in place and stay indoors". They should update family and friends of their status when possible, it adds.  

Continuing to read the military's statement, the TRT announcer says Turkey's democratic and secular rule of law has been eroded by the current government, Reuters reports. The country is now run by a "peace council" that will ensure the safety of the population, the announcer adds. A new constitution will be prepared "as soon as possible".

A statement has been out on state television. It is not clear who the statement was by, amid reports that the army chief was being held.

The BBC's Turkey correspondent - who is abroad - tweets:

The Turkish military has declared martial law and imposed a curfew, an announcer on state-run broadcaster TRT says.

Newspaper Hurriyet reports that President Recep Tayyip Erdogan is expected to make a statement soon. It is not known where he is.

Russian Foreign Minister Sergei Lavrov says it is important to "avoid bloodshed" in Turkey. Any issues should be settled within a constitutional framework, he adds.

Tanks are reportedly blocking Istanbul and Ankara airports. All flights from the main international airport in Istanbul have been cancelled.

Image source, AFP

Soldiers are inside the buildings of the Turkish state broadcaster TRT, one of its correspondents tells Reuters.   

Sources in Turkish President Recep Tayyip Erdogan's office tells Reuters and CNN Turk that he is safe. His whereabouts are unknown.