A US Senator has blocked a resolution to formally recognise the mass killing of Armenians by Ottoman Turks during World War One as a genocide.
Republican Senator Lindsey Graham said lawmakers should not "sugarcoat history or try to rewrite it".
Mr Graham's objection came hours after he attended a meeting with US President Donald Trump and Turkish President Recep Tayyip Erdogan.
The vote on the issue was viewed by Turkey as highly contentious.
To become official policy, the resolution needed to be approved by both houses of Congress and then be signed by the president.
It passed the House of Representatives by a vote of 405 to 11 in October, shortly after Turkey's military offensive against Kurdish fighters - allied to the US in fighting the Islamic State (IS) group - in northern Syria.
The resolution was blocked by Mr Graham on Wednesday when it reached the Senate. Under Senate rules, a single senator can block any resolution.
"I just met with President Erdogan and President Trump about the problems we face in Syria by the military incursion by Turkey. I do hope that Turkey and Armenia can come together and deal with this problem," he told the Senate floor.
Mr Graham is a staunch ally of President Trump, but has vocally criticised his administration for withdrawing troops from north-eastern Syria ahead of the Turkish military operation against Kurdish forces.
Last month Mr Erdogan described the House of Representatives vote as "worthless" and the "biggest insult" to Turkish people.
In contrast, Armenian Prime Minister Nikol Pashinyan hailed the move as "a bold step towards serving truth and historical justice".
There is general agreement that hundreds of thousands of Armenians died when the Ottoman Turks deported them en masse from eastern Anatolia to the Syrian desert and elsewhere in 1915-16. They were killed or died from starvation or disease.
The total number of Armenian dead is disputed. Armenians say 1.5 million died. The Republic of Turkey estimates the total to be 300,000. According to the International Association of Genocide Scholars (IAGS), the death toll was "more than a million".
Turkey denies that there was a systematic campaign to slaughter Armenians as an ethnic group during World War One.
The dispute about whether it was genocide centres on the question of premeditation - the degree to which the killings were orchestrated.
Many historians, governments and the Armenian people believe that they were, but a number of scholars question this.
Turkish officials accept that atrocities were committed but argue that there was no systematic attempt to destroy the Christian Armenian people. Turkey says many innocent Muslim Turks also died in the turmoil of war.