Here is a look at the rift between the NATO allies over the deal.
WHAT ARE S-400s?
The Russian-made S-400 Triumph is a sophisticated long-range surface-to-air missile defense system capable of striking enemy aircraft and cruise missiles. It has a range of 400 kilometers (250 miles). The system can simultaneously engage multiple targets and is capable of shooting down ballistic missile warheads along with aircraft and cruise missiles.
Turkey reached a deal with Russia for the purchase of two batteries at a reported cost of $2.5 billion in December 2017. Erdogan has said the systems will both be operational by April 2020. Officials have refused to disclose where the systems will be deployed.
WHY IS NATO-MEMBER TURKEY BUYING THE RUSSIAN SYSTEM?
Turkey, which neighbors trouble spots such as Syria, Iraq and Iran, has long sought to address shortcomings concerning its air defenses. It says it was forced to negotiate with Russia for the purchase of the S-400s after the U.S. refused to sell the American-made Patriot system. Turkey has also argued that the S-400 is one of the best available systems and says the deal with Russia involves joint production and technology transfers which meet its long-term goals of defense self-sufficiency.
The United States says talks on a potential Patriot deal failed over Turkey's insistence on technology transfer rights that would have allowed it eventually to make the missiles themselves. This ran against U.S. manufacturer's propriety interests in addition to any national security concerns.
Turkey's rapprochement with Russia and its decision to buy the Russian system also coincides with growing Turkish mistrust of the U.S. over its policies in Syria. More specifically, Turkey has been angered with U.S. support for a Syrian Kurdish group in Syria that is affiliated with Kurdish rebels fighting Turkey.
Turkey also distrusts Washington because Muslim cleric Fethullah Gulen, whom Ankara says orchestrated a 2016 failed coup, remains free in the U.S. Gulen denies involvement in the coup attempt.
WHY DOES THE US OBJECT TO TURKEY'S S-400 PURCHASE?
The U.S. says the S-400s can't be integrated into the NATO system and pose a threat to the F-35 fighter jet program. Washington is concerned that system could be used to gather data on the capabilities of the F-35, and that the sensitive information could end up in Russian hands.
Turkey insists the U.S. fighter jets won't be compromised because the S-400 and F-35s will be deployed in separate locations and the S-400 will be under Turkish control. Turkey has also proposed the setting up of a joint working group to study how the S-400 system would interact with the fighter jets.
WHAT DOES TURKEY'S SUSPENSION FROM THE F-35 PROGRAM MEAN?
Pentagon's chief weapons buyer, Ellen Lord, told a news conference that the U.S. has suspended Turkey from the F-35 program and is beginning the process of its formal removal. The process of removing Turkey permanently is underway and should be completed by March 31, she said.
Lord said Turkey, which makes more than 900 components of the stealth aircraft, stands to lose $9 billion in future earnings as its parts supplier.
Washington has already suspended a program of F-35 flight training for Turkish student pilots and instructor pilots at Luke Air Force Base in Arizona and given those personnel a July 31 deadline to leave the U.S.
Separately, the U.S. has also warned that Turkey could face economic sanctions under the Countering America's Adversaries Through Sanctions Act (CAATSA). Previous U.S. sanctions on Turkey over its detention of an American pastor had caused a slide in the Turkish lira.
However, President Donald Trump, who has struck cordial relations with Erdogan, has taken a more conciliatory approach to Turkey, raising hopes in the country that it will avert harsh penalties. The U.S. State Department this week has been silent on whether those sanctions will be imposed.
Turkey says it is prepared for U.S. sanctions and can impose countermeasures of its own. Some media reports have said Turkey could launch an operation into northeast Syria in reaction to the U.S. sanctions.
HOW DOES THE DEAL AFFECT NATO?
Turkey has been a key member of the NATO alliance since it joined in 1952. It has the second-largest army in NATO after the U.S. and has been protecting the alliance's southeastern flank for years.
It is unprecedented for a NATO ally to purchase such advanced defense weaponry from Russia, which is considered to be NATO's main adversary. Turkey's deal with Russia has raised questions over whether the country is moving under Russia's influence and over its future membership.
However, NATO Secretary General Jens Stoltenberg has said Turkey's contributions to NATO and the alliance's cooperation with Ankara "is much broader" than the F-35 issue. He said: "I'm not underestimating the difficulty related to (the) S-400, but I'm saying that Turkey, as a NATO member, is much more than S-400."
Turkish Defense Minister Hulusi Akar has said the purchase "was not an option but rather a necessity" because of Turkey's security concerns. He stressed that there was no change in Turkey's "strategic orientation."