Delivery of S-400 missile systems is continuing, Turkish National Defense Ministry said on Saturday.

Axar.az reports citing Anadolu Agency.

“Delivery of S-400 Long Range Air and Missile Defense Systems resumed today. The fourth Russian plane carrying S-400 parts landed at Murted Airport outside Ankara,” the ministry said on Twitter.

Following protracted efforts to purchase an air defense system from the U.S. with no success, Ankara signed a contract in 2017 to purchase the Russian S-400s.

U.S. officials urged Turkey to buy U.S. Patriot missiles, arguing the Russian system would be incompatible with NATO systems and expose the F-35s to possible Russian subterfuge.

Turkey, however, emphasized the S-400 would not be integrated into NATO systems and would not pose a threat to the alliance.

Turkey has urged formation of a commission to clarify any technical issues, but the U.S. has failed to respond to this proposal.

The U.S. has threatened sanctions over the purchase, with Turkey responding that any sanctions would be met in kind.