Turkey’s purchase of Russian S-400 air defense systems is a national security problem for NATO, since the deployment of F-35 fighter jets is impossible amid the Russian systems, Reuters reported referring to US officials.

According to them, the purchase of S-400 air defense systems was not tantamount to it withdrawing from NATO, but that Ankara’s purchase should be viewed as a national security issue, not a merely commercial decision.

“We are continuing to work on a range of options to ensure that Turkey’s participation in the NATO alliance and bilateral relationship can continue unabated and unimpinged,” one of the officials said. “The gravity of the risk to the F-35 both to the United States and to NATO allies is such that the two systems cannot be co-located.”

The US State Department last week said Washington had told Turkey that if it buys the S-400 systems, the US will have to reassess Ankara’s participation in the Lockheed Martin F-35 fighter program.

On Thursday, Erdogan repeated that it was not possible for Ankara to back out of the deal with Russia.