"You don't buy a product to keep it in a box," Foreign Minister Mevlut Cavusoglu told a press conference.

Turkey's acquisition of the S-400 missile defence system from Russia this summer was met with consternation by its NATO allies and threats of sanctions from Washington.

US officials had lately suggested Turkey could be spared sanctions -- automatic for countries that buy Russian weapons -- if it did not activate the system.

But Turkey tested the S-400 on Monday and Tuesday at a military base in Ankara province, using military jets including American-made F-16 fighters.

"It's concerning," US Secretary of State Mike Pompeo said Tuesday. "But we have made very clear to the Turkish government our desire to see them move away from the putting into full operationalisation of the S-400 weapons system."

The US says there is a risk that sensitive information about operational and technical capacities could be gleaned if the S-400 is used alongside Western equipment, especially the new F-35 jet.

Turkey has ordered 100 F-35s and its defence industry was part of the supply chain for the new jet, until it was kicked off the programme due to the S-400 purchase.

"We are partners in this project, we have made very serious investments," said Cavusoglu.

"In a worst case scenario, if we cannot buy the F-35 we will have to look for alternatives," he said, reiterating a long-held threat to look to other countries for new fighter jets.