In a regular visit to Washington last month, a senior Turkish opposition figure met with White House officials to talk about bilateral Turkish-American relations. Topics such as Syria and the now-resolved visa crisis were discussed, but another matter was more sensitive than he expected – Turkey's purchase of the Russian-made S-400 missile defense system.

Öztürk Yılmaz, the deputy chairman of the Republican People's Party (CHP), told journalists following his meeting that American officials, including Fiona Hill, President Trump's special assistant and senior director for Eurasia on the National Security Council, were seriously troubled by the Russian deal.

Sources in Washington, D.C. told me that the Trump administration was readying sanctions against Turkey over the sale. To solidify their message, a senior White House official told the CHP delegation that they wouldn't just stop with Turkey.

"American officials said they would also have to sanction Saudi Arabia if they go along with the preliminary deal with Moscow over a S-400 sale. They are pretty serious," the source told me.

Americans have repeatedly underlined that it is the Russia sanctions law that forces them to apply pressure on Turkey. The Countering America's Adversaries Through Sanctions Act, ratified by the U.S. Congress in July, mandates the U.S. president sanction people and companies that transact with Russia's defense sector.

According to the U.S. State Department, the manufacturer of S-400 systems, Almaz-Antey Air and Space Defense Corporation, is among the Russian state companies that people must not deal with.

Turkey and Russia finally acknowledged last week that they have signed a deal for two batteries of S-400 missiles reportedly for $2.5 billion. So to speak, the sanctions might be on Trump's agenda within a very short time.

Various sources told me that one of the most important factors that can push the administration to sanction is congressional pressure. So far we know at least Sen. Ben Cardin, the ranking Democrat on the Senate Foreign Relations Committee, has expressed his request directly to the White House in a letter that Turkey must be sanctioned.

I know as a fact that Turkey doesn't have many friends in the U.S. Congress who would resist such a demand. So, many lawmakers might have followed the same course and conveyed similar opinions to the administration.

This issue coincided with another problem, the so-called visa crisis between Turkey and the U.S., which finally ended when Washington abruptly decided to fully restore its consular services in Turkey last week. Many were convinced both in Ankara and Washington that the U.S. wouldn't end this crisis until Metin Topuz, the arrested Turkish staff member of the U.S. Consulate in Istanbul, was released. It didn't happen. Yet Washington decided to end the crisis, citing past Turkish assurances.

Multiple Turkish officials told me that the U.S. decision to end the crisis was a true surprise for them. An official said: "We really didn't understand what made them take this step. We think there will be more serious disagreements in the future between our two countries. And they just wanted to drop this from the agenda; otherwise it would be something irrevocable down the road."

Other officials believe the U.S. wanted to show the Turkish public that it didn't have any intention to hurt the Turkish people with this visa crisis, and their target was the Turkish government. Many people in Washington attribute the visa decision to the upcoming S-400 sanctions, saying Washington just wanted to end a long-due crisis that didn't serve anyone.

Speaking to a Turkish outlet, American sources said the current Charge D'Affaires in Ankara Phil Kosnett played a large role in taking back the visa decision. One administration source also told the outlet that the main reason to end the crisis was the fact that it overshot the target.

In any case, if Trump decides to act accordingly, he would have to sign a sanction order against the Turkish Undersecretariat for Defense Industries (SSM) for its S-400 purchase. This may prove costly since the SSM is jointly producing weapons with American companies, including the F-35 project.

An Anadolu Agency report suggested disqualifying the SSM from the F-35 project would cost $12 billion to the consortium since Turkey is planning to buy 120 jets. The SSM is also producing some of the hardware and is the engine maintenance contractor of the project.

Kasım İleri, the Pentagon correspondent for AA, told me that the U.S. would possibly sanction less important contracts and purchases such as munitions and software sales. He also reminded me that there were other costly projects such as the production of Sikorsky helicopters in Turkey.

What is done is done. The only solution to this problem is Trump issuing a waiver to Turkey on the grounds that it is in the national security interest of the United States. I wonder what Trump would ask from Turkey for this favor.