On Friday morning, video showed Russian planes unloading parts of the S-400 missile defense system in Turkey. For months, U.S. officials have warned Turkey -- a NATO ally -- that the Russian system is incompatible with and provides a security risk to the F-35 program, and that Turkey would be removed from the program should they choose to purchase the S-400.
Turkey also faces possible U.S. sanctions over purchasing Russian defense equipment under a law Congress passed in 2017 to force President Donald Trump's hand to be tougher on Moscow -- sanctions that Trump would not commit to imposing two weeks ago when meeting with Turkish President Recep Tayyip Erdogan.
The Pentagon announced on Friday morning that it would hold an on-camera briefing to discuss its response, but that briefing was delayed until the Pentagon told reporters it had been "postponed indefinitely."
Prior to his meeting with Uzbekistan's defense minister, Acting Defense Secretary Mark Esper told reporters on Friday the department was aware of Turkey taking the delivery of the missile defense system.
"Our position regarding the F-35 has not changed," he said. "I will speak with my Turkish counterpart Minister Akar this afternoon. So there will be more to follow after this conversation."
A U.S. defense official said that conversation between Esper and Akar lasted 30 minutes, with the S-400 delivery one of several topics of discussion. The Pentagon will not provide a readout of the call, but a Turkish readout reportedly said Akar called for a U.S. delegation to be urgently sent to Ankara next week to continue dialogue.
Top Republicans and Democrats on the Senate Foreign Relations and Armed Services Committee released a joint statement Friday afternoon, urging Trump to "fully implement sanctions as required by law" against Turkey for accepting the S-400. They also urged the Pentagon "to proceed with the termination of Turkey's participation in the F-35 program."
"It did not have to come to this. But unfortunately, President Erdogan rejected multiple attempts by the United States to preserve our strategic relationship," said Sen. Jim Risch (R-ID), Bob Menendez (D-NJ), Jim Inhofe (R-OK), and Jack Reed (D-RI).
"Lasting improvement to our cooperation will not be possible as long as President Erdogan remains fixated on deepening ties with Vladimir Putin at the expense of the economic prosperity of Turkey and the security of the NATO alliance," the statement added.
But when Trump was asked two weeks ago at the G-20 whether he would sanction Turkey over the S-400, he said it was a "complicated deal."
"We're working on it," the president said. "We'll see what we can do."
Trump has backed Erdogan in blaming the Obama administration for refusing to sell Turkey the U.S. Patriot missile defense system, which Trump argued forced Ankara to turn to Russia instead.
"It's a mess. It's a mess, and honestly, it's not really Erdogan's fault," Trump said at the G-20. "I think he was unfairly treated."
But that claim "is not true," said Risch, a Trump ally and top Republican on the Senate Foreign Relations Committee.
In 2013 in particular, he added, Turkey "had many opportunities to purchase our Patriot missiles over many years."
Morgan Ortagus, a spokeswoman for the State Department, said Tuesday that Turkey "will face real and negative consequences if they accept the S-400," including economic sanctions.
Months into Trump's term, Congress passed a law to sanction Russia, along with North Korea and Iran, for its 2016 election interference. The president, under political pressure for cozying up to Putin in Germany that July, begrudgingly signed.
Among other requirements, the Countering America's Adversaries Through Sanctions Act (CAATSA) required the administration to sanction any country that purchased equipment from a list of Russian defense or intelligence agencies or firms.
The law has only been used once, against China's defense procurement agency, which purchased Russian fighter jets and missiles systems. But the State Department has repeatedly warned that Turkey will face similar sanctions for its S-400 purchase.
The State Department did not respond to requests for comment Friday on whether it would implement sanctions.
Turkey will also be economically impacted from its loss of the F-35 program.
More than 900 parts found inside the F-35 are built in Turkey, as part of the original international consortium agreement for American allies to develop the F-35. About 400 of those parts, found in the aircraft's landing gear and central fuselage, are exclusively made by Turkish manufacturers.
Pentagon officials have already been looking to make arrangements to find alternate production facilities in other countries by early 2020.