The U.S. mission in Turkey warned Americans of "potential terrorist attacks and kidnappings" in Istanbul Friday in an urgent security alert.
The warning, which said U.S. citizens and other foreigners could be targeted, comes after U.S. forces warned of a threat from al Qaeda in Syria.
The alert said specifically that the U.S. consulate general in Istanbul was threatened, but it did not provide details on where the threats come from or who provided the reports.
As a precaution, all U.S. diplomatic facilities are closed to the public, including for American citizens, the alert said.
The alert was "a result of our ongoing assessment of security conditions," an embassy spokesperson told ABC News. "We are grateful for the support of the Turkish government in ensuring the safety of Americans living in Turkey as well as Turkish citizens who visit our Embassy and Consulates."
While the embassy and State Department declined to provide more details, the U.S. has recently increased its air strikes against al Qaeda operatives in neighboring Syria.
A meeting of senior al Qaeda leaders was targeted by U.S. forces on Thursday, U.S. Central Command said in a statement.
"AQ-S takes advantage of the instability in northwest Syria to establish and maintain safe havens to coordinate terrorist activities," CENTCOM warned.
U.S. officials have been warning about the strength of Islamist forces in Idlib province, the last rebel stronghold in Syria after nearly a decade of war. Syrian opposition forces continue to beat back an assault by strongman Bashar al Assad with support from Turkey and its military, but some of those fighters include hard-line jihadists.
As the fight against ISIS continues, the U.S. military has increasingly turned to targeting al Qaeda-linked operatives, too -- with Thursday's strike the second in as many weeks.
ABC News's Engin Bas contributed to this report.