Turkish President Recep Tayyip Erdogan says his country is seeking to acquire the Russian S-500 missile defense system, a more advanced version of the S-400, taking a dispute with the United States up a notch.

Speaking at a televised question and answer session with university students in Istanbul on Saturday, Erdogan stressed that Turkey’s purchase of Russia�s S-400 system was “a done deal” and that Ankara was absolutely not “stepping back on the S-400 issue” under US threats.

“The S-400 deal is completed. Under the contract, the deliveries are to begin in July or, maybe, even earlier. Russia has offered us very good conditions. After that, we will talk about the S-500, including joint production [of these systems], as well as S-400,” he said.

Turkey has signed a 2.5-billion-dollar deal with Russia for the purchase of S-400 system. That contract is severely opposed by the US, which has tied the implementation of a contract of its own with Turkey — for the sale of 30 American F-35 stealth fighter jets — to the cancellation of the deal with Russia.

Washington has also gone as far as to warn Ankara that it could lose its NATO membership.

Last month, the Pentagon halted the delivery of equipment related to the F-35 to Turkey.

‘US halts F-35 equipment shipment to Turkey’

The United States has reportedly stopped shipment of equipment for F-35 equipment to Turkey

Erdogan, however, said that Turkey would “sooner or later” get the F-35s despite the US delay in the process.

“They (the Americans) are passing the ball around in the midfield now, showing some reluctance. But sooner or later, we will receive the F-35s. Not delivering them is not an option,” he said.

The US has claimed that the Russian defense system is not interoperable with NATO equipment.

But President Erdogan said Turkey had conducted technical studies on the compatibility of the S-400s and the F-35s and had found no issues of concern as expressed by the US.

Recently, Foreign Minister Mevlut Cavusoglu said Turkey would look elsewhere for an alternative to the F-35 if the US blocked the delivery of its advanced stealth warplanes to Ankara.

The remark by President Erdogan about a potential purchase of the S-500 is certain to stir the dispute with the US.