WASHINGTON — Clouding the NATO leaders meeting in London this week is Turkey's decision to buy and begin testing of a Russian missile system — a move that is believed to pose a risk to the world's strongest military alliance.

Ankara faces blowback from NATO members as well as U.S. sanctions for completing its messy multibillion-dollar arms deal with the Kremlin.

Yet Turkey's government has shrugged off the potential sanctions since accepting delivery of the Russian S-400 missile system, and President Donald Trump himself has been sympathetic to Turkish President Recep Tayyip Erdogan's decision.

Last month, the two leaders met in Washington with seemingly no breakthrough on the S-400 issue.

Earlier this year, Trump said he could not blame Turkey for buying the S-400, a move that resulted in dropping the NATO ally's participation in the F-35 program.

"I don't blame Turkey because there are a lot of circumstances and a lot of ... problems that occurred during the Obama administration," Trump said in July. "This dates back to the Obama administration, which was a disaster."

Read more: Turkey proceeds with deal for Russian missile system despite US and NATO warnings

Trump then did not elaborate on whether he would impose sanctions on Turkey for doing business with the Kremlin. Under the Countering America's Adversaries Through Sanctions Act, or CAATSA, which the president signed in August 2017, Turkey could face economic sanctions for buying the Russian-made missile system.

"It's a tough situation. They're getting the S-400 and our statues and everything else — as you do that, you just can't order this equipment," Trump said. "And generally speaking, you can't order equipment period."

In efforts to deter Turkey from buying the S-400, the State Department offered in 2013 and 2017 to sell the country Raytheon's Patriot missile system. Ankara passed on Patriot both times because the U.S. declined to provide a transfer of the system's sensitive missile technology. Turkey can still get the Patriot, but only after meeting certain conditions.

"There is room for Turkey to come back to the table. They know that to make this work they need to either destroy, return, or somehow get rid of the S-400. At the same time, we certainly have not closed the door on their ability to acquire the Patriot battery, which does address their air defense needs," explained a senior State Department official, who spoke on the condition of anonymity.

The official also said that the timeline for imposing CAATSA sanctions "is not prescribed or absolute."

"There is still plenty of scope that could be applied as to where sanctions and the breadth and depth of sanctions could be imposed upon Turkey," the official added.