WASHINGTON (Reuters) - U.S. lawmakers pressed the Trump administration on Monday to impose sanctions on Turkey over its purchase of a Russian missile defense system, saying the failure to do so sends a “terrible signal,” and a key Senate committee set a vote on additional legislation to punish Ankara.

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“The time for patience has long expired. It is time you applied the law,” Democratic Senator Chris Van Hollen and Republican Senator Lindsey Graham said in a letter to Secretary of State Mike Pompeo. “Failure to do so is sending a terrible signal to other countries that they can flout U.S. laws without consequence,” they said.

Ankara and Washington have been at loggerheads over NATO ally Turkey’s purchase of the Russian S-400 system, which Washington says is not compatible with NATO defenses and poses a threat to its F-35 stealth fighter jets, which Lockheed Martin Corp is developing.

Infuriating many members of Congress, Turkey shrugged off the threat of U.S. sanctions and began receiving its first S-400 deliveries in July. In response, Washington removed Turkey from the F-35 program.

U.S. President Donald Trump’s administration has held off on imposing sanctions despite Trump signing a sweeping sanctions law, known as CAATSA, in 2017 mandating them for countries that do business with Russia’s military.

U.S. lawmakers’ anger toward Turkey deepened after Ankara crossed into Syria for an offensive against Kurdish militias that had helped U.S. forces combat Islamic State militants.

Senator Jim Risch, the Republican chairman of the Foreign Relations Committee, told reporters the committee would vote next week on a separate bill to impose stiff sanctions on Ankara.

Asked if that meant his patience with Turkey had worn thin, Risch told reporters that that had happened a while ago.

An aide said the Foreign Relations Committee would consider the bill, co-sponsored by Risch and Senator Bob Menendez, the committee’s top Democrat, on Wednesday.

Normally ardent defenders of fellow-Republican Trump, Graham and some other party members have been harshly critical of the president’s decision to withdraw troops from northeastern Syria, paving the way for the Turkish move against Kurdish fighters.

Trump hosted his Turkish counterpart, Tayyip Erdogan, at the White House for a meeting last month that Trump described as “wonderful.” But there has been no sign that Erdogan has changed his plan to buy the Russian system.

The State Department did not respond to a request for comment. Pompeo said on Nov. 26 that Turkey carrying out tests on the Russian system was “concerning,” and that talks to resolve the issue were still under way.

The same day, Russia’s state arms exporter Rosoboronexport was cited as saying Moscow hoped to seal a deal to supply Turkey with more S-400 missile systems in the first half of 2020.