Turkey will be sanctioned if it goes forward with the purchase of Russia's S-400 missile system, acting U.S. Defense Secretary Mark Esper told his Turkish counterpart Hulusi Akar on Wednesday, June 26 following a NATO defense ministerial, Deutsche Welle reports.

"Turkey has been a longstanding and trusted partner and ally for many, many years," Esper said. "The pursuit of the S-400 undermines that."

For months, US officials have urged Turkey to scrap its deal to acquire the Russian-made aerial missile defense system and buy the American-made Patriot system. Esper said Turkey would be dropped from the F-35 fighter jet program if it went ahead with the deal.

Turkey has refused to bow down to U.S. pressure, saying the deal for the S-400s has been completed. Deliveries are expected in July.

Read more: Is NATO's future at risk over U.S.-Turkey rift?

'Imposing sanctions on each other'

Before leaving to Japan for a G20 summit, Turkish President Recep Tayyep Erdogan warned Washington against imposing sanctions on a fellow NATO ally.

"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 is scheduled to meet with U.S. President Donald Trump on Saturday. The acquisition of Russian-made military hardware is likely to be a contentious subject for the two.