US President Joe Biden on Saturday recognised the 1915 killings of Armenians by Ottoman forces as genocide, a watershed moment for descendants of the hundreds of thousands of dead as he defied decades of pressure by Turkey.

Biden became the first US president to use the word genocide in a customary statement on the anniversary, a day after informing Turkish President Recep Tayyip Erdogan that he would go ahead with this step and seeking to limit the expected furor from the Nato ally.

“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,” Biden said.

“And we remember so that we remain ever-vigilant against the corrosive influence of hate in all its forms,” he said.

The statement is a massive victory for Armenia and its extensive diaspora.

Starting with Uruguay in 1965, nations including France, Germany, Canada and Russia have recognised the genocide but a US statement has been a paramount goal that proved elusive under other presidents until Biden.

Biden said his statement was “not to cast blame but to ensure that what happened is never repeated”.

Biden made the decision “in a very principled way focused on the merits of human rights, and not for any reason beyond that, including placing blame,” a senior US official said.

Biden took office vowing to put a new focus on human rights and democracy in the wake of his volatile predecessor Donald Trump, who befriended authoritarians and, despite breaking plenty of foreign policy precedents, declined to recognise the Armenian genocide.

Explaining Biden's thinking, the administration official also alluded to the Democratic president's outspokenness on systemic racism in the United States.

Across the world, “people are beginning to acknowledge and address and grapple with the painful historical facts in their own countries. It's certainly something that we are doing here in the United States,” she said.

Turkey accuses US of trying to 'rewrite history'

Meanwhile, Turkey accused the United States of trying to rewrite history, resoundingly rejecting Biden's decision to formally recognise the Armenian genocide.

“Words cannot change or rewrite history,” Turkish Foreign Minister Mevlut Cavusoglu tweeted moments after Biden announced his decision. “We will not take lessons from anyone on our history.”

In a message to the Armenian patriarch in Istanbul, Erdogan also accused “third parties” of trying to politicise the century-old debate.

“Nobody benefits from the debates — which should be held by historians — being politicised by third parties and becoming an instrument of interference in our country,” Erdogan wrote.

But on a more conciliatory note, Erdogan said Turkey was “ready to develop our relations with Armenia based on good neighbourhood and mutual respect”.

Cavusoglu's foreign ministry struck a more strident tone.

“We reject and denounce in the strongest terms the statement of the president of the US regarding the events of 1915 made under the pressure of radical Armenian circles and anti-Turkey groups on 24 April,” the foreign ministry said in a separate statement.

“It is clear that the said statement does not have a scholarly and legal basis, nor is it supported by any evidence,” it said.

“With regards to the events of 1915, none of the conditions required for the use of the term 'genocide' that is strictly defined in international law are met.”

A century of waiting

As many as 1.5 million Armenians are estimated to have been killed from 1915 to 1917 during the waning days of the Ottoman Empire, which suspected the Christian minority of conspiring with adversary Russia in World War I.

Armenian populations were rounded up and deported into the desert of Syria on death marches where many were shot, poisoned or fell victim to disease, according to accounts at the time by foreign diplomats.

Turkey, which emerged as a secular republic from the ashes of the Ottoman Empire, acknowledges that 300,000 Armenians may have died but strongly rejects that it was genocide, saying they perished in strife and famine in which many Turks also died.

Erdogan on Thursday told advisors to “defend the truth against those who back the so-called 'Armenian genocide' lie,” with his foreign minister warning that the United States would set back relations.

Recognition has been a top priority for the Armenia and Armenian-Americans, with calls for compensation and property restoration over what they call Meds Yeghern — the Great Crime.

Biden's statement was also expected to heighten appeals from Armenia for greater US support against Turkish-backed neighbor Azerbaijan, which last year humiliated Armenia by taking back swathes of territory seized in the 1990s.

But Biden, whose call to Erdogan to inform him of the genocide recognition was their first conversation since the US leader took office three months ago, has signaled he hopes for limited diplomatic impact.

Biden and Erdogan agreed in their call to meet in June on the sidelines of a NATO summit in Brussels, officials said. Biden has kept Erdogan at arm's length — a contrast with Trump, whom the Turkish leader reportedly found so amenable that he would call Trump directly on his phone on the golf course.

Turbulent alliance

The US Congress in 2019 voted overwhelmingly to recognise the Armenian genocide but the Trump administration made clear that the official US line had not changed.

Former president Barack Obama, under whom Biden served as vice president, danced around the issue by referencing pre-election statements he made recognising the genocide and resisted pressure for a statement on the centennial in 2015.

Alan Makovsky, an expert on Turkey at the left-leaning Center for American Progress, said that the 2019 congressional resolution had “no discernible impact” on US-Turkey relations — and paved the way for Biden to go ahead.

“We've seen through experience that concern about Turkey's reaction was always overblown,” he said.

“Turkey will raise a rhetorical fuss for a few days and perhaps delay acting on some routine requests from the US military.”

Tensions have risen with Turkey in recent years over its purchase of a major air defense system from Russia — the chief adversary of Nato — which under US law could trigger sanctions.

Turkey has also infuriated much of the US political establishment with its incursions against US-allied Kurdish fighters in Syria who helped fight the Islamic State group but are linked to militants inside Turkey.

Biden before taking office called Erdogan an autocrat and voiced support for Turkey's opposition. His administration has also criticised homophobic statements from those close to Erdogan.