On Thursday, the CEO of Russia's state-owned Rostec defense company said the deal with Turkey exceeded $2 billion, according to Tass, the Russian news agency. It followed Turkish President Recep Tayyip Erdogan in September announcing the longtime member of NATO had made a down payment to Moscow for the S-400 and that he and Vladimir Putin, the Russian president, "are determined on this issue."
"Don't be surprised if that deal for the S-400 system never actually sees the full light of day," said Richard Safran, a defense analyst at Buckingham Research in New York. "The Turkish situation on the surface appears to be degrading right now."
Regardless, the potential sale of the S-400 has been one of the irritants in U.S.-Turkish relations and also raised questions about Ankara's long-term motivations and its commitment to the NATO alliance.
Last week, Gen. Petr Pavel, chairman of the NATO Military Committee, said during an event in Washington that Turkey should it go ahead with the acquisition would be "facing the consequences of that decision."
Turkey wants to manufacture the S-400 domestically in a joint venture with Russia as part of an overall effort by Ankara to build the country's military manufacturing expertise. However, that arrangement would mean the Russians would have to share the technology with Turkey, and all indications so far are Moscow probably wouldn't allow the transfer.
Analysts say the sale has other problems too, including the Russian military equipment's lack of interoperability with NATO's armed forces. No NATO ally currently operates the S-400.
"I do not think it will happen," said Luke Coffey, director of the Allison Center for Foreign Policy at the Heritage Foundation, the Washington-based conservative think tank. First, he believes Ankara is using the Russians as leverage to get a better deal on an air-defense system from the U.S. or a European ally. Second, he doesn't believe Moscow will transfer the S-400 technology to Turkey.
Indeed, Turkish paper Hurriyet last month reported a Putin advisor, Vladimir Kojin, had said Russia would not provide the technology transfer sought by Ankara. Also, Turkish Foreign Minister Mevlut Cavusoglu was quoted by the Istanbul paper Aksam as saying: "If Russia doesn't want to comply, we'll make an arrangement with another country."
Wolfe Research analyst Hunter Keay said in a note last month that Erdogan "overruled" the comments by his foreign minister and also "pointed out having talks over the still-in-development S-500 missile system," a newer version of Russia's defense system.
Even so, Russian media have still played up Turkish interest in the S-400 for many months, noting that China was a buyer of the system. More recently, it's been reported that Saudi Arabia is interested in the air defense system.
Back in 2015, Turkey looked set to acquire China's export version of the FD-2000 long-range air-defense missile system for $3.4 billion but backed out after pressure from NATO. Also, the Chinese reportedly refused to budge on all of the Ankara government's technology-transfer demands.
"We're not expecting Turkey to switch course again, but further estrangement from the U.S./NATO and closer ties with Russia could create risks around existing programs like the F-35," said Keay.
Turkey is scheduled to get 100 F-35 stealth fighter jets, with the first aircraft delivery expected to happen in 2019.
Despite strains in the U.S.-Turkish alliance, analysts say the controversial Erdogan probably is playing to a domestic audience with the Russian hardware purchase since it fits into his strategy to continue consolidating power and demonstrating regional superpower status regardless of NATO.
"Turkey is doing a lot of posturing to show that it is capable of going elsewhere besides NATO for its weapons and its alliances," said Loren Thompson, a defense industry consultant and chief operating officer of the Lexington Institute, a Virginia-based public-policy think tank supported by U.S. defense giants.
Since a failed coup attempt in 2016, Erdogan's government has detained several Americans. Erdogan blames the West for helping alleged coup plotters and their sympathizers.
Other challenges in the U.S.-Turkish relationship stem from the U.S. alliance with the YPG-led Syrian Democratic Forces to defeat ISIS. The Kurdish YPG militia are a branch of the outlawed terrorist group PKK (or Kurdistan Workers' Party), which is accused of killing some 1,200 Turkish security forces and civilians.
Additionally, there's been a diplomatic dispute between Ankara and Washington that led Erdogan last month to accuse the U.S. of unfairly treating its longtime ally. There's also been a diplomatic spat and suspension of visa services to each other's citizens.
Meantime, Erdogan and Putin have worked to mend a relationship that was rocked after Turkey shot down a Russian fighter jet in late 2015. Putin has visited Turkey at least two times since ties between the two countries resumed last year.
"Maybe Erdogan and Putin get along personally, but these countries have been competitors and enemies on and off through the course of centuries," said Coffey. "The idea that they will all of a sudden become these close buddies goes against the grain of history."
Added Coffey, "Erdogan tries to play one off the other to advance Turkey's position in the region. He'll cooperate with Russia when he thinks it suits it, or then he'll go back to NATO and the U.S."