According to TASS, during a press conference, the head of the US State Department was asked to comment on the joint testing by Turkey of the S-400 and F-16 fighters.
“Yes, it is a concern,” said Pompeo.
Earlier, US Senator Chris Van Hollen said that Turkey crossed the "red line" by starting testing the S-400 radar.
The Turkish media reported that the State Department demanded that Turkey "get rid of the S-400." Otherwise, Washington threatened Ankara with new sanctions.