"It's concerning," Pompeo told reporters when asked about reports that Turkey used US-developed F-16 fighter jets to test the S-400 system controversially purchased from Moscow.

But he added: "We are hopeful. We're still talking to the Turks. We're still trying to figure our way through this thing."

"We have made very clear to the Turkish government our desire to see them move away from the putting into full operationalization of the S-400 weapons system," he said.

Turkey defied US warnings to buy the weapons system from Russia, which culminated in Ankara's expulsion from the F-35 fighter-jet program.

US officials fear that Turkey's purchase will let Russia fine-tune the S-400's ability to counter Western jets and missiles.

But US officials have said that Turkey can reverse course by returning the S-400, destroying it or essentially keeping it in its box.

President Donald Trump has held off on imposing sanctions on Turkey, as required by a 2017 law that aims to punish any country with "significant" military transactions with Russia.

Pompeo declined to comment on what Trump "may or may not do" on the sanctions.

The S-400 purchase came during a year of high tensions between the United States and Turkey.

Last month, Trump briefly slapped sanctions on senior Turkish officials over Ankara's assault on Kurdish fighters in Syria, an incursion made possible by Trump's decision to withdraw US troops.