The US will send 31 powerful battle tanks to Ukraine, joining Germany in sending the vehicles to support the fight against Russia's invasion.
The decision to deliver the M1 Abrams tanks was announced just hours after Germany said it would send 14 of its Leopard 2 tanks to the battlefield.
Berlin also cleared the way for other European countries to send German-made tanks from their own stocks.
Ukraine has lobbied Western allies to send the military equipment for months.
It hailed the twin announcements as a turning point that would allow its military to regain momentum and take back occupied territory almost a year after Moscow invaded. It also said the tanks could help deter a potential Russian offensive in the spring.
"An important step on the path to victory," Ukraine's President Volodymyr Zelensky said. "Today the free world is united as never before for a common goal - liberation of Ukraine."
Russia, meanwhile, condemned the moves as a "blatant provocation" and said any supplied tanks would be destroyed. "These tanks burn like all the rest. They are just very expensive," President Vladimir Putin's spokesman said.
"Putin expected Europe and the United States to weaken our resolve," President Joe Biden said while announcing the decision at the White House on Wednesday. "He was wrong from the beginning and he continues to be wrong."
"We're also giving Ukraine the parts and equipment necessary to effectively sustain these tanks in battle," he said. "This is about helping Ukraine defend and protect Ukrainian land. It is not an offensive threat to Russia."
A Ukrainian tank battalion typically consists of 31 tanks, which is why that number has been agreed upon, Mr Biden said.
The US decision, however, marks a reversal in their position as the Biden administration has insisted for some time that the heavy M1 Abrams tanks would be difficult to deliver, expensive to maintain and challenging for Ukrainian troops to operate.
The US-made military vehicle is one of the most modern battle tanks in the world and requires extensive training to operate. The $400m (£323m) American package also includes eight recovery vehicles that can tow the tanks if they become stuck.
But it is likely to be many months before the tanks reach the battlefield, experts say, as they will be purchased from private contractors and not sent from an existing stockpile.
The German-made Leopard 2 tanks, however, will be drawn from existing stock and are expected to arrive in two to three months. They are widely seen as one of the most effective battle tanks available.
The decision to send the heavy weapons follows weeks of diplomatic wrangling. Germany faced mounting international pressure to send the tanks, and there are reports that the eventual decision to do so was conditional on the US doing the same.
Both sides participated in "good diplomatic conversations" that made the difference and contributed to the "extraordinary shift in Germany's security policy", a senior US official said on condition of anonymity earlier on Wednesday.
When asked if the US decision was designed to give Germany cover to send tanks, national security spokesman John Kirby said: "I wouldn't use the word cover. What this decision does do is show how unified we are with our allies."
He attributed the change in Washington's position to the conditions on the ground as well as Russia's tactics, without giving further details.
Mr Kirby also said the decision was based on the "kinds of fighting... that we believe the Ukrainians are going to need to be capable of in the weeks and months ahead".
Ukrainian crews would soon be trained to use the Leopard tanks in Germany, officials in Berlin said. Mr Biden said troops would be trained to use the American-made tanks "as soon as possible" but added that delivering them would take time.
While the acquisition of tanks from the West will be considered a diplomatic coup for President Volodymyr Zelensky, he said on Tuesday that his country required at least 300 of them to defeat Russia.
Several European countries have Leopard 2 tanks in their stocks, and the German decision means some of these can also be sent to Ukraine. Germany hopes around 90 will ultimately be delivered to the battlefield.
Norway announced later on Wednesday that it would send Leopard 2 tanks to Ukraine, but did not specify how many it would deliver.
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