Dozens of countries allied with Ukraine will meet on Friday to coordinate arms supplies for Kyiv, which has ramped up its requests to be provided with tanks.
Germany faces growing pressure to allow its highly regarded Leopard tanks to be exported to Ukraine.
The crunch talks come a day after the US and several European nations pledged more equipment to help Ukraine fend off any further Russian offensives.
The UK also offered 14 of its battle tanks recently – but Kyiv wants more.
Ukraine’s allies faced a “simple choice” on Friday, an adviser to the defence minister remarked ahead of the summit.
“Tanks for Ukraine are tanks for freedom,” Yuriy Sak told BBC’s Newsnight programme. If these were not sent, other countries might one day “have to use them themselves” against Moscow, he warned.
More than 50 countries are due to meet at the US Ramstein Air Base in Germany’s south-west.
German Chancellor Olaf Scholz faces mounting pressure to donate Leopard 2 tanks, following the UK’s recent donation of 14 of its own Challenger battle tanks.
As the nation of manufacture, Germany must also give its permission before other countries can commit to sending their own supplies of the Leopard.
The Polish prime minister said he was “moderately pessimistic” about getting Berlin’s blessing – though a Lithuanian minister told the AFP news agency that “some countries” would send the Leopards.
Late on Thursday, Germany said its position would “become clear in the next few hours or tomorrow morning”, AFP reported.
Ukrainian President Volodymyr Zelensky has repeatedly taken aim at Berlin’s perceived hesitancy, saying on Thursday that he was still “waiting for a decision from one European capital”.
He also hopes to receive a number of American-made Abrams tanks, and addressed reports that the US and Germany were only planning to commit vehicles if the other nation did the same.
“I don’t think this is the right strategy to go with”, Mr Zelensky commented, while calling for “strong decisions” to be made during Friday’s conference.
Retired US Army general David Petraeus suggested there was “legitimate reluctance” on the part of Washington, which resulted from concerns over the maintenance of these vehicles.
He told the BBC it was “imperative” that any Western tank donations were made “early enough, so [Ukrainian soldiers] can actually train on them”.
For its part, Russia has warned Western countries that providing tanks to its enemy would mark an “extremely dangerous” escalation in the conflict.
On Thursday, Western nations pledged to send more vehicles, artillery and munitions to bolster the Ukrainian war effort.
The US committed a new package worth $2.5bn (£2bn), saying this took its spend on Ukrainian support to $26.7bn since last February’s full-scale invasion by Russia.
Tanks were not included in the offer, but the Pentagon did promise an extra 59 Bradley armoured vehicles, 90 Stryker personnel carriers and Avenger air defence systems, among other provisions.