WASHINGTON, April 12 (CNA) - President Joe Biden on Tuesday (Apr 12) accused Russian President Vladimir Putin's forces of committing genocide in Ukraine, the first time his administration has used the term.

"Yes, I called it genocide," Biden told reporters travelling with him in Iowa when asked about his use of the term during a speech earlier in the day.

"It's become clearer and clearer that Putin is just trying to wipe out the idea of even being able to be a Ukrainian," said the US leader.

Biden said it would ultimately be up to courts to determine whether Russia's actions in its ex-Soviet neighbour - where it stands accused of atrocities against civilians - constitute genocide.

"We'll let the lawyers decide internationally whether or not it qualifies, but it sure seems that way to me," he said.

Biden added: "The evidence is mounting.

"More evidence is coming out of literally the horrible things that the Russians have done in Ukraine.

"And we're going to only learn more and more about the devastation."

Ukraine has been accusing Russia of committing war crimes since even before the discovery of hundreds of civilians reportedly killed in Bucha sparked an outpouring of revulsion.

Biden described Putin as a "war criminal" amid the global outrage and called on him to face trial over the alleged atrocity.

But the United States has stopped short of using the term "genocide," in line with longstanding protocol, because of its strict legal definition and the heavy implication the accusation carries.

Biden first levelled the accusation at Putin during a speech about soaring gasoline prices earlier on Tuesday, saying Americans' ability to fill up their tank should not "hinge on whether a dictator declares war and commits genocide half a world away".

The Biden administration has sought to blame sharp rises at US gas stations on Putin's invasion.

Biden had previously been asked by reporters whether the killings in Bucha amounted to "genocide" and he replied: "No, I think it is a war crime."

Under international law, genocide is an intent to destroy - in whole or in part - a national, ethnic, racial or religious group.

According to UN convention, this includes through killings; serious bodily or mental harm; inflicting lethal conditions and measures to prevent births, among other means.

Biden has made a handful of statements about the war that US officials have later had to walk back. The president stirred controversy on a recent trip to Poland when he ad-libbed a line at the end of a speech and said that Putin should not be allowed to remain in power.

The White House clarified that US policy was not to seek regime change.

Genocide, considered the most serious international offence, was first used to describe the Nazi Holocaust. It was established in 1948 as a crime under international law in a United Nations convention.

Since the end of the Cold War, the State Department has formally used the term seven times. These were to describe massacres in Bosnia, Rwanda, Iraq and Darfur; the Islamic State’s attacks on Yazidis and other minorities; China’s treatment of Uighurs and other Muslims and this year over the Myanmar army's persecution of the Rohingya minority. China denies the genocide claims.

At the State Department, such a determination normally follows a meticulous internal process. Still, the final decision is up to the secretary of state, who weighs whether the move would advance American interests, officials said.