Theoretically the sanctions were to be automatic, mandated by Congress, if NATO member Turkey opted to buy the S-400 system from NATO's main adversary Russia.

But since Ankara began taking delivery of the system, designed to protect the country from air attacks, on July 12, President Donald Trump has signaled he is loath to punish the US ally economically over the deal.

Legislators from both parties want the US leader to show firmness, after having pressed Turkey hard for two years not to follow through.

But after meeting with Trump this week, Republican senators were not optimistic.

"It was certainly clear that the president is not in a place right now where he wants to impose sanctions on Turkey," said an assistant to one of the legislators in the White House meeting, speaking on condition of anonymity.

The billionaire president, who doesn't mask his affinity for controversial autocrats, says he has a close relationship with President Recep Tayyip Erdogan.

And the Turkish strongman has claimed that this relationship will permit him to escape sanctions.

The S-400 is viewed as threat to NATO -- it could be used to take aim at US and NATO aircraft.

For months Washington has repeatedly warned of "very serious" sanctions "if it completes the delivery of the S-400."

However the message from Washington has changed since then. The United States' F-35 fighter jet program "cannot coexist with a Russian intelligence collection platform that will be used to learn about its advanced capabilities," the White House said last week.

But on signing off on the sanctions, Trump has demurred.

"We're not looking at that right now," he said on July 18.

"I've had a good relationship with President Erdogan," he said. "We're working through it -- we'll see what happens."

- Semantic twists -

"There is no timetable" in the so-called CAATSA legislation requiring sanctions for the S-400 purchase, State Department spokeswoman Morgan Ortagus said Thursday, stressing that the action taken on the F-35 program was already "very, very heavy."

The 2017 CAATSA legislation requires punitive economic measures against any country engaging in "a significant transaction" with the Russian military.

But since the delivery took place, Washington has subtly shifted. Secretary of State Mike Pompeo has now said that the red line is the "activation" of the missile defense system.

"The activation of the S-400 is unacceptable," he told Bloomberg TV on Thursday.

"There could be more sanctions to follow, but frankly what we'd really like is the S-400 not to become operational."

And Senator Lindsey Graham, an ally of the president, told the Defense One publication that he is mediating between the two sides.

He said that he had told Ankara that if they don't activate the S-400, "the sanctions don't have to be applied."

In exchange, Graham said, he offered to begin free trade agreement negotiations with Turkey -- an approach Pompeo did not deny.

Erdogan has played along -- he said the S-400 won't be operational until April, prolonging the poker game between the two countries.

Yet some experts say the system is a threat even when it isn't fully activated.

The senator's aide acknowledged an ongoing debate about what constitutes "a significant transaction," but said that, ultimately, the Trump administration has a legal requirement under CAATSA to sanction Turkey.

Pressure is still coming from other parts of Congress, with some lawmakers questioning Turkey's future in the Atlantic Alliance.

"NATO was created as a bulwark against Russian aggression," said Senator Rick Scott.

"I have serious concerns about allowing Turkey to continue to enjoy the protections of NATO while cozying up to Moscow."