Both of the main bridges in the Turkish city of Istanbul have been closed by security forces, reports say. Traffic has been stopped from crossing both the Bosphorus and Fatih Sultan Mehmet bridges, but no reasons have been given.
There are also reports of military aircraft flying over the capital Ankara, with gunshots heard.
It is unclear if the events are related and there has been no official information so far on the deployments.
Turkey's PM has denounced an "illegal action" by a military "group", with bridges closed in Istanbul and aircraft flying low over the capital, Ankara.
Binali Yildirim said the military action was not authorised but it was not a coup. He said that the government remained in charge.
Traffic has been stopped from crossing both the Bosphorus and Fatih Sultan Mehmet bridges in Istanbul.
Mr Yildirim told NTV television by telephone: "We are working on the possibility of an attempt. We will not allow this attempt." He did not elaborate.
"Those who are in this illegal act will pay the highest price," he added, saying it would not be correct to describe the move as a "coup".
Turkey's military coups
- 1993 - Claims of a "covert coup" intended to prevent a peace settlement with the Kurdistan Workers' Party (PKK)
- 1980 - Military coup following armed conflict between right-wing and left-wing groups in the 1970s
- 1971 - Military coup known as the "coup by memorandum", which the military delivered instead of sending out tanks
- 1960 - Coup by group of young military officer outside chain of command, against the democratically-elected Democrat Party