The U.S. State Department expressed serious concerns about Turkey’s potential acquisition of Russia-made S-400 air-defence system.

In response to a question by a journalist, a US State Department spokesperson Heather Nauert said that the system being purchased from Russia is not compatible with NATO systems, including the F-35 fighter jet.

“I can tell you that we have had serious concerns about Turkey’s potential acquisition of the S-400 system” – repeated Heather Nauert.

More: TASS: Russia begins delivering S-400 to China

“Under NATO and under the NATO agreement, which of course, Turkey is a NATO member, you’re only supposed to buy, they are only supposed to buy, weapons and other materiel that are interoperable with other NATO partners. We don’t see that as being interoperable,” the State Department spokeswoman also noted during a press briefing.

More: Russia deploys more S-400 air defence systems to Syria

Earlier in April, Assistant Secretary of State Wess Mitchell warned that Ankara’s decision to buy Russia’s advanced complexes exposes Turkey to possible US sanctions and may bar it from getting F-35 jets. “Ankara claims to have agreed to purchase the Russian S-400 missile system, which could potentially lead to sanctions” under the Countering America’s Adversaries Through Sanctions Act (CAATSA), he said. The document was signed back in August 2017 and is mostly aimed at hindering Russian arms exports.