Russia flew a fresh shipment of advanced air defense equipment to Turkey on Saturday, the Turkish Defense Ministry said, despite Washingtons warnings that it will impose sanctions on Ankara. The ministry said a fourth Russian cargo plane landed at the Murted air base near the Turkish capital, a day after three huge Russian air force AN-124 planes offloaded equipment at the base. Washington has tried for months to prevent the deal, arguing that the Russian S-400 air defense system is incompatible with NATO systems. It also says that if the S-400s are deployed near US F-35 jets, which Turkey is buying and helping to produce, they would undermine the stealth fighter planes defenses. US officials had warned that Turkey would be thrown off the F-35 program if it took delivery of the S-400s, and would also face sanctions under US legislation seeking to prevent countries from buying military equipment from Russia. Turkey says S-400 is a strategic defense requirement, above all to secure its southern borders with Syria and Iraq. Reaction from Washington was limited on Friday, with acting Defense Secretary Mark Esper saying the US stance had not changed. Esper later spoke with Turkish counterpart Hulusi Akar. While the Pentagon declined to discuss the call, a Turkish Defense Ministry statement said “Akar told his US counterpart that Turkey remains under a serious air and missile threat and that purchase of S-400 defense systems was not an option but rather a necessity."