The United States embassy in Turkey has been closed to the public as of Monday due to a security threat and only emergency services will be provided, the mission said in a statement.
The embassy advised US citizens in Turkey to avoid large crowds and the embassy building, and to be aware of their own security when visiting popular tourist sites and crowded places.
It did not specify what the security threat was that prompted the closure.
Additional security measures were taken after intelligence from US sources suggested there might be an attack targeting the US embassy or places US citizens were staying, the Ankara governor's office said in a statement on Sunday.
Visa interviews and other routine services would be cancelled on Monday, the embassy said, adding that it would make an announcement when it was ready to reopen.
The embassy was the target in 2013 of a suicide bombing, claimed by a far-left group that killed a Turkish security guard.
Relations between NATO allies Washington and Ankara have been strained over a number of issues, notably the US arming of a Kurdish fighters in northern Syria, Kurdish People's Protection Units (YPG).
Ankara on January 20 launched an operation against the YPG in its border stronghold of Afrin in northwestern Syria.
Turkey considers the Kurdish Democratic Union Party (PYD) in Syria and its armed wing YPG to be "terrorist groups" with ties to the outlawed Kurdistan Workers' Party (PKK), active in Turkey.
The PKK has waged a decades-long armed fight against the Turkish state that has killed tens of thousands of people.
Turkey has repeatedly called on Washington to stop working with the YPG.
The US, in return, has expressed concern about the Turkish Operation dubbed "Olive Branch" and urged restraint from Turkey.