WASHINGTON (Reuters) - The U.S. Senate on Thursday unanimously passed a resolution that recognizes as a genocide the mass killings of Armenians a century ago, a historic move that infuriated Turkey and dealt a blow to the already problematic ties between Ankara and Washington.

FILE PHOTO: U.S. President Donald Trump and Turkey's President Tayyip Erdogan pose for a family photo during the annual NATO heads of government summit at the Grove Hotel in Watford, Britain December 4, 2019. REUTERS/Peter Nicholls/Pool

Turkey condemned the measure, which passed a month after an official visit to the White House by Turkish President Tayyip Erdogan, who enjoys a special rapport with President Donald Trump, amid mounting issues that have soured the relationship between the two NATO allies.

Trump had cast his Nov. 13 meeting with Erdogan as “wonderful” despite no concrete breakthrough on deep disagreements such as Ankara’s purchase of Russian weapon systems and diverging views on Syria policy.

The Democratic-led House of Representatives had passed the resolution by an overwhelming majority in October. But a vote in the Senate was repeatedly blocked by Trump’s fellow Republican senators since meeting with Erdogan.

“This is a tribute to the memory of 1.5 million victims of the first #Genocide of the 20th century and bold step in promotion of the prevention agenda. #NeverAgain,” Armenian Prime Minister Nikol Pashinyan tweeted in English.

The resolution asserts that it is U.S. policy to commemorate as genocide the killing of 1.5 million Armenians by the Ottoman Empire from 1915 to 1923. The Ottoman Empire was centered in present-day Turkey.

Turkey accepts that many Armenians living in the Ottoman Empire were killed in clashes with Ottoman forces during World War One, but contests the figures and denies that the killings were systematically orchestrated and constitute a genocide.

Turkish foreign minister Mevlut Cavusoglu called the decision a “political show” while presidential spokesman Ibrahim Kalin said Ankara strongly condemned and rejected the measure. The resolution is nonbinding.

“History will note these resolutions as irresponsible and irrational actions by some members of the US Congress against Turkey,” Fahrettin Altun, Turkey’s communications director, said on Twitter in response.

VINDICATION

Congressional aides said the White House does not want the legislation to move ahead while it negotiates with Ankara on sensitive issues. However, since the visit, Erdogan repeatedly said Turkey has no intention of dropping the Russian S-400 air defense missile systems it has bought, crushing any hopes for progress.

For decades, measures recognizing the Armenian genocide have stalled in the U.S. Congress, stymied by concerns about relations with Turkey and intense lobbying by Ankara.

“I’ve invested like decades of my life,” said Aram Hamparian, executive director of the Armenian National Committee of America. “So it was a sense of relief and a bit of a vindication that ... (the United) States recognized the history of the Armenians, but also put up a firewall against foreign countries coming into our democracy and dictating to us.”

The U.S. Congress has been united in its opposition to Turkey’s recent policy actions. Republican senators have been incensed with Turkey’s purchase of the S-400, which the United States says pose a threat to its F-35 fighter jets and cannot be integrated into NATO defenses.

They have also moved to punish Turkey over its Oct. 9 incursion into Syria. A U.S. Senate committee backed legislation on Wednesday to impose sanctions on Turkey, pushing Trump to take a harder line on the issue, as many lawmakers blame Trump for giving a green light to Ankara for its military offensive.

To become law, that legislation would have to pass the House of Representatives, which passed its own Turkish sanctions bill by an overwhelming 403-16 vote in October, and be signed by Trump.