The White House on Friday put a halt to a Pentagon briefing on U.S. sanctions to be imposed on Turkey after it received the first delivery of Russian S-400 missile systems, Breaking Defence news site reported citing U.S. officials.

The first batch of Russian defense systems arrived at an airbase in Ankara on Friday. The delivery of the S-400 systems will continue with two more shipments and some 80 Turkish servicemen will receive training in Russia in July and August, state-run Anadolu news agency reported.

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Washington has concerns that Turkey’s acquisition of S-400 systems will damage NATO’s interoperability and expose the F-35 fighter jets to possible Russian subterfuge.

The U.S. Department of Defense (DoD) last month set July 31 as the deadline for Turkey’s expulsion from the joint production program of F-35 stealth fighters, if Ankara goes ahead with plans to acquire S-400 systems. There is also bipartisan support in the U.S. Congress for further sanctions to be imposed on Turkey under the Countering America’s Adversaries Through Sanctions Act (CAATSA).

According to Breaking News, the Pentagon had twice pushed back a planned briefing on Friday by senior officials on sanctions to be imposed on Ankara. The news site said it was unclear when anyone from the US government would comment.

“Officials said that the White House stepped in and put a halt to DoD’s carefully planned briefing, which had been scheduled for this (Friday) morning,” Breaking News said.

Acting U.S. Defense Secretary Mark Esper said on Friday that the United States had not changed its position on Turkey’s role in the F-35 program if they accepted a Russian missile defense system and he would be speaking with his Turkish counterpart later in the day.

Esper spoke with Turkish Defence Minister Hulusi Akar for 30 minutes on Friday, but the Pentagon did not provide a readout of the conversation, Breaking Defence said. “That is usually a strong indicator that the two sides had little on which they could agree,” the news site said.

Turkish President Recep Tayyip Erdoğan said last month after a meeting with U.S. President Donald Trump on the sidelines of G-20 summit in Osaka that Trump had assured Turkey would not be subject to U.S. sanctions over the purchase of S-400 batteries.

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Trump, before his meeting with Erdoğan, said that Turkey had been treated unfairly as Ankara was not allowed to buy U.S.-made Patriot batteries by the former U.S. administration under President Barack Obama.

“Sanctions will go to the White House, but Trump is the wild card and could sit on implementation,” Aaron Stein, the director of the Foreign Policy Research Institute’s Middle East program, told Foreign Policy on Thursday.