Joe Biden has become the first US president to issue a statement formally describing the 1915 massacre of Armenians as a genocide.
The killings took place in the waning days of the Ottoman Empire, the forerunner of modern-day Turkey.
But the issue is highly sensitive, with Turkey acknowledging atrocities but rejecting the term “genocide”.
Foreign Minister Mevlut Cavusoglu said on Saturday that Turkey “entirely rejects” the US decision.
“We will not take lessons from anyone on our history,” he tweeted.
Later the Turkish foreign ministry said it had summoned the US ambassador to convey Ankara’s “strong reaction”.
Previous US administrations have not used the term genocide in formal statements amid concerns over damaging relations with Turkey, a Nato ally.
What happened in 1915?
Ottoman Turks had accused Christian Armenians of treachery after suffering a heavy defeat at the hands of Russian forces and began deporting them en masse to the Syrian desert and elsewhere. Hundreds of thousands of Armenians were massacred or died from starvation or disease.
Atrocities were widely recorded at the time by witnesses including journalists, missionaries and diplomats.
The number of Armenian dead has always been disputed. Armenians say about 1.5 million people died. Turkey estimates the total to be closer to 300,000. According to the International Association of Genocide Scholars (IAGS), the death toll was “more than a million”.
Although Turkish officials have accepted that atrocities took place, they argue that there was no systematic attempt to destroy the Christian Armenian people. Turkey says many Muslim Turks also died in the turmoil of World War One.
What did Biden say?
Mr Biden’s statement, released as Armenia commemorates the start of the mass killings, said: “We remember the lives of all those who died in the Ottoman-era Armenian genocide and recommit ourselves to preventing such an atrocity from ever again occurring.
“And we remember so that we remain ever-vigilant against the corrosive influence of hate in all its forms.”
Mr Biden said the intention was “not to cast blame but to ensure that what happened is never repeated”.
He had previously welcomed a move by the US House of Representatives, which in 2019 voted overwhelmingly to recognised the mass killings as a genocide.
A Biden official told reporters that the decision to use the term formally as the administration turned its focus to human rights.
In 1981, then-President Ronald Reagan referred to the “Armenian genocide” in a proclamation on the Holocaust, but others have shied away from using the term since.
The administration of Mr Biden’s immediate predecessor Donald Trump said it did not consider the killings a genocide. Mr Trump instead called it “one of the worst mass atrocities of the 20th Century”.
What has the reaction been?
Armenian Prime Minister Nikol Pashinyan said Mr Biden’s words had “honoured the memory” of those who had died, adding in a tweet: “The US has once again demonstrated its unwavering commitment to protecting human rights and universal values.”
But the Turkish foreign ministry responded angrily, saying in a statement they “reject and denounce in the strongest terms the statement”, saying it had been “made under the pressure of radical Armenian circles and anti-Turkey groups”.
It warned the move would “open a deep wound that undermines our mutual trust and friendship”.
A further deterioration of relations between the two countries may be the most significant outcome of Mr Biden’s statement, which is largely symbolic and comes with no additional sanctions.