The US Department of State on Thursday updated its travel advisory for Turkey amid stalled US-Turkey talks on Syria and growing differences over Venezuela and Turkish S-400 deal with Russia.

Turkey announced Thursday it would be going ahead with a purchase of Russia’s S-400 missile system, ahead of an informal Friday deadline to respond to a rival US offer.

Turkey, a NATO member, made repeated moves to buy the Russian missile defence system, despite warnings from the US-led alliance that the S-400s cannot be integrated into the NATO air defence system.

The State Department upped the advisory to “Level 3”, urging citizens “to reconsider travel the Middle Eastern country due “to terrorism and arbitrary detentions” and to refrain from going to “the Syria and Iraq borders."

“Terrorist groups continue plotting possible attacks in Turkey. Terrorists may attack with little or no warning, targeting tourist locations, transportation hubs, markets/shopping malls, local government facilities, hotels, clubs, restaurants, places of worship, parks, major sporting and cultural events, educational institutions, airports, and other public areas,” the advisory read.

It also made reference to mounting detentions by Turkish authorities, which increased this week after the government rounded up more than 700 individuals suspected of having ties to the network of US-based cleric Fethullah Gulen.

“Security forces have detained tens of thousands of individuals, including U.S. citizens, for alleged affiliations with terrorist organizations based on scant or secret evidence and grounds that appear to be politically motivated” the US statement read. It also warned that US citizens have also been subject to travel bans that prevent them from departing Turkey.

The US government urged against “travel near the Turkey/Syria and Turkey/Iraq borders due to the continued threat of civil war in Syria and attacks by terrorist groups.”

The warning comes as US and Turkey continue their consultations over establishing a safe zone within Syria’s border. Differences over policing the zone and the future of the Syrian Democratic Forces have hampered those plans.

US special representative to Syria James Jeffrey is expected to attend the Munich Security conference this week where he is expected to hold meetings with his Turkish counterparts. Turkish President Recep Tayyip Erdogan took part on Thursday in a trilateral summit in Russia with Vladimir Putin and Iranian President Hassan Rouhani, both considered rivals of Washington in Syria.