Turkey would retaliate if the United States imposes sanctions over its purchase of Russian S-400 missile defense system, Turkish President Recep Tayyip Erdogan said on Thursday.

In a meeting with foreign press in Istanbul, Erdogan urged the United States to be sensitive to sanctions against Turkey "which is a NATO member."

"Imposing sanctions on one of the most powerful members of the bloc is not acceptable," he said, stressing that Turkey would not remain silent on US punitive measures.

"We will have sanctions of our own," he added.

The Turkish leader also stated that the purchase of S-400s was "a done deal" and their first delivery will be made during the first half of July to locations previously determined by the Turkish Armed Forces.

Erdogan said he would discuss the issue with his US counterpart Donald Trump during the G20 summit slated for late June in Japan.

Washington is reportedly halting the training of Turkish pilots of US F-35 fighter jets in addition to threatening economic and other military sanctions as Ankara insists on its S-400 deal with Russia.

Turkey has paid 1.25 billion US dollars so far for F-35s, Erdogan said, adding his country would appeal to an international court for a refund in case of a "false step" by Washington.