Turkey carrying out tests with the Russian defense system it purchased was “concerning,” U.S. Secretary of State Mike Pompeo said on Nov. 26, but added that talks between Washington and Ankara to resolve the issue were still underway.

Speaking at a news conference, Pompeo said the United States has made it clear to Turkey that Washington wants to see Ankara move away from full operation of the S400 air defense system.

“Yes it is concerning,” Pompeo said, when he was asked about the reports that Turkish F-16 warplanes flew over the country’s capital Ankara on Nov. 25 to test its new Russian S-400 missile system.

“We are hopeful. We are still talking to the Turks, still trying to figure out our way through this thing,” he said.

Ankara and Washington have been at loggerheads over Turkey’s purchase of the S-400 system, which Washington says is not compatible with NATO defenses and poses a threat to its F-35 stealth fighter jets, which Lockheed Martin is developing.

U.S. President Donald Trump hosted his Turkish counterpart Recep Tayyip Erdoğan at the White House earlier this month for a meeting, which Trump described as “wonderful.” However, it was not clear if the two NATO allies made any breakthrough on the S-400 impasse.

‘S-400s will not be integrated into NATO systems’

Meanwhile, Turkish presidential spokesman İbrahim Kalın said on Nov. 26 that Turkey will not integrate Russian S-400 missile systems into the NATO security or air defense systems.

“We are still working on the technical details. The S-400 systems will not be integrated into the NATO security system or air defense system,” Kalın told a foreign policy conference in Berlin.

“It will remain an independent defense system on its own. Concerns on this issue can be eased,” he said.

Earlier on Nov. 26, the head of Russian state arms exporter Rosoboronexport was quoted as saying Russia hoped to seal a deal to supply Turkey with more S-400 missile systems in the first half of next year.