The Biden administration is poised to approve sending M1 Abrams tanks to Ukraine, US officials have said.
It is a further sign that international reluctance to send tanks to the battlefront against the Russians has started to erode.
The decision could be announced as soon as Wednesday, though it could take months or years for the tanks to be delivered.
US officials said details are still being worked out. One said the tanks would be bought under an upcoming Ukraine Security Assistance Initiative package, which provides longer-range funding for weapons and equipment to be purchased from commercial vendors.
The US announcement is expected in co-ordination with an announcement by Germany that it will approve Poland's request to transfer German-made Leopard 2 tanks to Ukraine, according to one official.
By agreeing to send the Abrams at an as-yet unspecified time under the assistance initiative, the administration is able to meet German Chancellor Olaf Scholz's demand for an American commitment without having to send the tanks immediately.
Much of the aid sent so far in the 11-month-old war has been through a separate programme drawing on Pentagon stocks to get weapons more quickly to Ukraine.
But even under that programme, it would take months to get tanks to Ukraine and to get Ukrainian forces trained on them.
It is unknown how many tanks would be approved.
Until now, the US has resisted providing its own M1 Abrams tanks to Ukraine, citing extensive and complex maintenance and logistical challenges with the high-tech vehicles.
Washington believes it would be more productive to send German Leopards since many allies have them and Ukrainian troops would need less training than on the Abrams.
Just last week, under secretary of defence for policy Colin Kahl told reporters that the Abrams is a complicated, expensive, difficult to maintain and hard-to-train-on piece of equipment.
One thing defence secretary Lloyd Austin has been very focused on, he said, "is that we should not be providing the Ukrainians systems they can't repair, they can't sustain, and that they, over the long term, can't afford, because it's not helpful".
At the Pentagon, spokesman Brigadier General Pat Ryder said he had nothing to announce on any US decision regarding Abrams tanks.
But he said "any time that we've provided Ukraine with a type of system, we've provided the training and sustainment capabilities with that".
The administration's reversal comes just days after a coalition of more than 50 senior defence officials from Europe and beyond met in Germany to discuss Ukraine's war needs, and battle tanks were a prime topic.
Ukrainian leaders have been urgently requesting tanks, but Germany had resisted mounting pressure either to supply its own tanks or clear the way for other countries, such as Poland, to send the German-made tanks from their own stocks.
Kremlin spokesman Dmitry Peskov said the deployment of Western tanks would trigger "unambiguously negative" consequences.
On Sunday, Berlin indicated it would not stand in the way if other countries wanted to send the Leopard 2 tanks to Kyiv. Germany needs to agree for the tanks to be given to Ukraine, which is not a member of Nato.
US and German officials have given mixed signals about whether the US and German decisions are linked, and whether Berlin was hesitant to send its tanks unless the US sent Abrams.
Polish defence minister Mariusz Blaszczak said Tuesday that Poland has officially requested permission from Germany to transfer its Leopard 2 battle tanks to Ukraine.
German officials confirmed to the dpa news agency they had received the application and said it would be assessed "with due urgency".
German officials declined to comment on the reports of a tank deal.