BRUSSELS, Belgium (AP) — Acting US Defense Secretary Mark Esper made it clear to Turkey on Wednesday that it will face economic sanctions if it goes ahead with the purchase of a Russian missile defense system, as the standoff between the two NATO allies dragged on.

Esper met with Turkish Defense Minister Hulusi Akar during a NATO meeting, delivering a familiar US message: If Turkey buys the Russian S-400 system, it will not be allowed to buy the high-tech F-35 fighter jet. The meeting, however, ended with no discernible results.

A senior defense official traveling with Esper said the two men had a frank and candid discussion, but the meeting resulted in no movement or change of position on either side. The official spoke on condition of anonymity to discuss a private meeting.

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Turkey has said that it won’t bow to US ultimatums and that the S-400 purchase is a done deal. The US has repeatedly said that the Russian system is a threat to the fighter jet and that Turkey will be dropped from the F-35 program if it buys the S-400. US officials have encouraged Turkey to buy the American-made Patriot missile defense system instead.

Speaking to reporters traveling with him to the NATO meeting, Esper, who took over as acting defense chief on Monday, said his message is pretty straightforward. “Turkey has been a longstanding and trusted partner and ally for many, many years,” he said. “The pursuit of the S-400 undermines that.”

The US has already stopped training Turkish pilots on the F-35, and given Ankara until the end of July to get its personnel out of the US.

US officials said Esper also told Akar during the meeting that until Turkey takes possession of the Russian system, there’s an opportunity to turn things around.

The session between Esper and Akar is viewed as a precursor to the expected meeting between US President Donald Trump and Turkish President Recep Tayyip Erdogan during the Group of 20 meeting in Japan later this week.

Erdogan told reporters in Ankara on Thursday that Trump in their discussions has not given him the impression that the United States would impose sanctions on Turkey. He repeated that the deal with Russia has been sealed and that Turkey was now waiting to take delivery of the S-400 systems.

“If NATO allies are now imposing sanctions on each other, I don’t know anything about that,” Erdogan said. “These are not impressions I got from the talks I have had with Mr. Trump until now.”

Erdogan has said that the training of Turkish personnel on the S-400 is finished and that the delivery of the system will take place in the first 15 days of next month. He said he also hopes to use the Trump meeting to dissuade the US from imposing sanctions.