The United States Senate on Friday unanimously adopted a resolution recognising the early 20th century killings of Armenians by Ottoman Turks as “genocide”, a label Turkey rejects.

Turkey condemned the measure, saying it will not help improve relations with the US. The two NATO allies are already at loggerheads over several issues, including Ankara’s purchase of Russian missile defense systems and policy over Syria.

“By passing my Armenian Genocide resolution, the Senate finally stood up to confirm history,” said Democratic senator Bob Menendez, who was one of the co-authors of the legislation.

The Democratic-led House of Representatives passed the resolution by an overwhelming 405-11 in late October. But a vote in the Senate, where President Donald Trump‘s fellow Republicans hold a majority of seats, had been blocked several times by Republican senators.

Fahrettin Altun, the Turkish presidency’s communications director, tweeted that the “behavior of some members of the U.S. Congress is damaging the Turkish-American ties”.

“History will note these resolutions as irresponsible and irrational actions by some members of the US Congress against Turkey,” he added, referring to Thursday’s measure and a bill that would impose sanctions on Turkey that made it through a Senate committee on Wendesday. “They will go down in history as the responsible party for causing a long lasting damage between two nations.”

History will note these resolutions as irresponsible and irrational actions by some members of the US Congress against Turkey. They will go down in history as the responsible party for causing a long lasting damage between two nations. — Fahrettin Altun (@fahrettinaltun) December 12, 2019

Armenians say up to 1.5 million of their kin were killed between 1915 and 1917 as the Ottoman Empire was falling apart.

They say the mass killings amounted to “genocide”, a claim recognised by about 30 countries.

Turkey strongly denies the accusation of “genocide” and says Armenians and Turks died as a result of World War I. It puts the death toll in the hundreds of thousands from both sides.

During his visit in Washington, DC, in mid-November, Turkish President Recep Tayyip Erdogan reiterated his call for historians to investigate the issue.

“If the US side really wants to act fairly, it should refrain from taking a political stand on a matter that historians should decide,” Erdogan said.

Some US politicians have been angered by recent actions by Turkey, including Ankara’s purchase of Russia’s S-400 missile defence system and its October offensive in northeast Syria against Kurdish forces.