Trump National Security Advisor John Bolton has blasted the S-400 deal as one of several major stumbling blocks in US-Turkish relations.
"We're concerned about their purchase of the Russian air defence system called the S-400 – that's a big problem," Bolton said, speaking to AM 970 radio host John Catsimatidis on Sunday morning.
Asked directly by Catsimatidis whether Turkey was a "friend or foe" to the United States, the top Trump foreign policy adviser cited a host of other problems affecting relations.
"Well you know they're still a NATO ally; we're trying to work with them, but they've got a very bad relationship with our close friends in Israel. That's something we need to look out on," Bolton said, adding that disagreements "with respect to the conflict in Syria" were another issue.
Ankara and Moscow penned a $2.5-billion contract on the delivery of four battalion sets of S-400s to Turkey in 2017, with the advanced air defence system expected to start being delivered this year. Washington has pressured Ankara to back away from the deal, and offered the country a $3.5 billion Patriot air defence system contract in place of the S-400s.
Turkey has ignored the requests, and insisted that it still expects to receive the first of its US-made F-35 fighter jets by November, despite attempts by the Pentagon to lobby Congress to block the sale of F-35s to Turkey if it went ahead with its S-400 purchase.
Last week, Pentagon spokesman Charlie Summers warned that "there would be grave consequences" for the US-Turkish military relationship if Ankara didn't cancel the S-400 deal. "They will not get the F-35s if they take the S-400s," the official said.
US officials have repeatedly called on Ankara to scrap the S-400 deal and called the possible deployment of S-400s in areas where F-35s are set to operate a "threat," presumably because the system would allow Turkey to test just how formidable the F-35's stealth systems are when matched up against the Russian air defence system.