After months of reluctance, the US and Germany reportedly plan to send tanks to Ukraine, in what Kyiv hopes will be a game-changer on the battlefield.
US President Joe Biden's administration is expected to announce plans to send at least 30 M1 Abrams tanks.
German Chancellor Olaf Scholz has also reportedly decided to send at least 14 Leopard 2 tanks. He is set to speak in parliament on Wednesday morning.
Russia's ambassador to the US said the news was "another blatant provocation".
And the Kremlin spokesman said Germany's reported decision would "bring nothing good" and leave "a lasting mark" on relations with Russia.
Ukrainian officials say they are urgently in need of heavier weapons, and say sufficient battle tanks could help Kyiv's forces seize back territory from the Russians.
But until now, the US and Germany have resisted internal and external pressure to send their tanks to Ukraine.
Washington has cited the extensive training and maintenance required for the high-tech Abrams.
Germans have endured months of painful political debate amid concerns that sending tanks would escalate the conflict and make Nato a direct party to the war with Russia.
US media is reporting that an announcement regarding Abrams shipments to Ukraine could come as soon as Wednesday, with unnamed officials cited as saying at least 30 could be sent.
However the timing remains unclear, and it could take many months for the US combat vehicles to reach the battlefront.
German officials had reportedly been insisting they would only agree to the transfer of Leopard 2s to Ukraine if the US also sent M1 Abrams.
"If the Germans continue to say we will only send or release Leopards on the conditions that Americans send Abrams, we should send Abrams," Democratic Senator Chris Coons, a Biden ally, told Politico on Tuesday.
Britain has already said it will send Challenger Two tanks to Ukraine.
Poland - one of 16 European and Nato countries that has German-made Leopard 2 tanks - has been pushing to send the vehicles to Ukraine, but under export rules needs Berlin's permission.
Ukraine is still unlikely to get the 300 modern main battle tanks it says it needs to win the war.
But if half a dozen Western nations each provide 14 tanks, then that would bring the total to nearly 100 - which could make a difference.
Western tanks - including the UK's Challenger 2, Germany's Leopard 2 and the US-made Abrams - are all seen as superior to their Soviet-era counterparts, like the ubiquitous T-72.
They will provide Ukrainian crews with more protection, speed and accuracy.
But Western modern main battle tanks are not a wonder weapon or game-changer on their own. It's also what's being supplied alongside them.
In recent weeks, there's been a step change in heavy weapons being supplied by the West - including hundreds more armoured vehicles, artillery systems and ammunition.
Combined together, they are the kind of military hardware needed to punch through Russian lines and to retake territory.
If Ukrainian troops can be trained and the weapons delivered in time, they could form key elements of any spring offensive. A missing element for offensive operations is still air power.
Ukraine has been asking for the West to provide modern fighter jets since the war began. So far, none has been delivered.
While there has been no official statement from the German government, Marie-Agnes Strack-Zimmermann of the liberal FDP party, who chairs the defence committee of the German parliament, welcomed the reports.
"The decision was tough, it took far too long, but in the end it was unavoidable," she said, adding that it would come as a relief to "the battered and brave Ukrainian people".
Allied nations had become frustrated at what they perceived as German reluctance to send the armoured vehicles in recent days.
German Defence Minister Boris Pistorius earlier said that Berlin had given other nations the green light to train Ukrainians to use Leopard 2 tanks, but did not commit to sending their own.
The Ukrainian president's chief of staff, Andriy Yermak, on Tuesday called on Western countries to give Kyiv hundreds of tanks to form a "crushing fist" against Russia.
"Tanks are one of the components for Ukraine to return to its 1991 borders," he wrote on Telegram.
Anatoly Antonov, the Russian ambassador to Washington, wrote on Telegram: "If the United States decides to supply tanks, then justifying such a step with arguments about 'defensive weapons' will definitely not work.
"This would be another blatant provocation against the Russian Federation."