WASHINGTON – Two senior US senators – Republican Lindsey Graham of South Carolina and Democrat Chris Van Hollen of Maryland – sent a letter to Secretary of State Mike Pompeo on Monday, pressuring him to sanction Turkey over its purchase of a Russian anti-aircraft weapon system."You have previously acknowledged that Turkey's purchase of the Russian S-400 air defense system triggers US sanctions," the two wrote in their letter. "Specifically, you said, ‘the law requires that there be sanctions. And I'm confident that we will comply with the law, and [US] President [Donald] Trump will comply with the law.’ "You were right when you made that statement," the senators' letter continued. "The Countering America's Adversaries through Sanctions Act requires the United States to impose sanctions on Turkey for its acquisition of the Russian-made S-400 system."Van Hollen and Graham added that in recent days, a Turkish defense official indicated that Turkey has gone beyond acquisition and is now testing a component of the S-400 system in Ankara. In addition, the two wrote that the head of Russian state arms exporter, Rosoboronexport, asserted that Moscow hopes to seal the deal to supply Turkey with even more S-400 missile systems in the first half of next year."Both the administration and Congress have repeatedly warned [Turkish] President [Tayyip] Erdogan that his decision to move forward with the S-400 requires the imposition of American sanctions," they further wrote. "Despite President Erdogan's recent visit to the White House, he has given no indications that he is going to change course."They concluded the letter by calling on Pompeo to act. "The time for patience has long expired," they wrote. "It is time you applied the law. Failure to do so is sending a terrible signal to other countries that they can flout US laws without consequence."