The US diplomatic mission in Turkey's largest city of Istanbul has suspended its consular services following a security threat. The consulate has not given details of the specific nature of the threat received.

Warning American citizens to avoid the Istanbul diplomatic post, the consular officials said their visa appointments and other non-emergency services would be rescheduled. A statement on the consular website read: "We strongly encourage US citizens to maintain a high level of vigilance, be aware of local events, and take the appropriate steps to bolster your personal security."

Other US embassies in the Turkish cities of Ankara, Izmir and Adana will operate normally. The skeleton diplomatic crew at the Istanbul compound would still be available for any emergency service.

Security measures around the diplomatic post in Istanbul have been heightened. The road leading to the mission has been cordoned off and metal barricades have been placed outside. Several police vehicles are stationed in front of the post.

"This supersedes anything that went before. Obviously, we want people to be mindful of the threat around the consulate in Istanbul. And clearly, send a security message that people should take pains to avoid that area," State Department spokesperson John Kirby told reporters.

An earlier security message warning Americans to avoid the US consulate in Istanbul until 6 December has been revised with the latest closure. The consulate has been targeted before in attacks carried out by extremists. Turkey, which shares its borders with the war-torn Syria, has been battling Kurdish separatists for long and has recently intensified its fight against the Islamic State (Isis) as well.