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.
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.
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 recognise 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.
Previous US administrations have not used the term genocide in formal statements amid concerns over damaging relations with Turkey, a Nato ally.
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.
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”.
Later, the Turkish foreign ministry said it had summoned the US ambassador to convey Ankara’s “strong reaction”.
It warned the move would “open a deep wound that undermines our mutual trust and friendship”.
But 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.”