"Upon intelligence coming to our units from U.S. sources that terrorist actions could be undertaken targeting the U.S. Embassy and where U.S. citizens are staying, security measures have been reviewed and extra measures have been taken", the Governor's Office said.
Policemen stand guard outside the United States embassy in Ankara, on March 5, 2018, as part of security measures set after the embassy was closed to the public over a "security threat".
Police secure roads around U.S. Embassy in Ankara on Monday.
The embassy said it would make an announcement when it was ready to reopen.
Their competing policies in war-ridden Syria such as Washington's continued aid for Kurdish forces fighting IS, and Ankara's ongoing invasion of the besieged enclave of Afrin in Syrian Kurdistan as well as the Turkish leadership's realignment with Russian Federation has added to the strained relations.
The Anadolu Agency said police detained four Iraqi nationals in connection with the threat against the embassy.
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In addition, the Ankara Chief Prosecutor's Office said police have detained 12 suspected ISIL militants in Ankara and are searching for eight others.
All suspects were foreign nationals, although officials did not specify their nationalities.
On March 5, Deputy Prime Minister Bekir Bozdağ said USA consulates in Turkey were "operating as normal".
It issued the same statement for Tuesday after the police detention of suspected IS members.
Those detained were foreigners who were trying to recruit members for IS, Anadolu reported, adding that the suspects were in contact with those in "conflict zones".
The Governorate of the Turkish capital separately warned of an attack on the USA embassy or public venues where Americans frequent. An attack at a luxury Istanbul nightclub during New Year celebrations on January 1, 2017, killed 39 people.