As Washington puts pressure on its NATO ally Turkey to reject a S-400 missile deal from Russia, Turkey has remained steadfast in its commitment to the agreement. 

"There is a certain step that we took, there is an agreement here and we are committed to it," Turkish President Recep Tayep Erdogan told reporters Tuesday after attending prayers due to the Muslim holiday of Eid al-Fitr. "It is out of the question that we step back."

The United States is alarmed that Turkey is buying the missiles from Russia. In April, four U.S. Senators wrote an op-ed for the New York Times delivering an ultimatum to Turkey that it will have to choose between Russia and the United States when it comes to fulfilling its defense needs. 

"By the end of the year, Turkey will have either F-35 advanced aircraft on its soil or a Russian S-400 surface-to-air missile system. It will not have both," Senators James Inhofe (R-Okla), Jack Reed (D-R.I), Jim Risch (R-Idaho) and Bob Menendez (D-N.J.) wrote.

The United States may block sales of U.S-made F-35 jets and Patriots missiles to Turkey due to the deal with Russia.  The U.S. government is concerned that if Turkey buys S-400 missiles and F-35s it could allow the Russians to get valuable intel on the fighter jet. 

 There has also been concern among officials in the Turkish government over the deal. 

 "The officials opposing the immediate delivery of the S-400 are concerned about ties with NATO being disrupted, U.S. sanctions and (the prospect) that the Patriots will be totally unavailable," Reuters reported. 

Sanctions have caused a major rift in the U.S.-Turkish relationship. Last year, the U.S. government called for the release of Andrew Brunson, an evangelical American pastor who was imprisoned in Turkey due to terrorism charges. The Turkish government refused to release Brunson. In retaliation, the U.S. government under President Donald Trump imposed sanctions and tariffs on Turkey in August 2018, sending the Turkish currency into a free fall.

In October, the Turkish government released Brunson. The fraught U.S. and Turkish relationship may be one reason why Turkey is now turning towards the Russians in regards to its defense policy.