Biden spoke before meeting NATO chief Jens Stoltenberg and eastern European leaders in a bid to shore up support for Kyiv nearly a year after Russia invaded Ukraine.
Asked by a journalist for a reaction to Russia's decision, Biden at first joked in Warsaw that he didn't "have time", then said: "big mistake".
The leaders of Bulgaria, the Czech Republic, Estonia, Hungary, Latvia, Lithuania, Poland, Romania and Slovakia attended the talks with Biden in Warsaw amid widespread concern in their countries that the conflict could spill over.
Stoltenberg urged allies to "step up our support for Ukraine" saying that "we cannot allow Russia to continue to chip away at European security".
In Moscow, President Vladimir Putin attended a patriotic rally, telling a crowd of tens of thousands that Russia was fighting "for our historical lands, for our people".
Earlier, he held talks with China's top diplomat Wang Yi, who was visiting Moscow after Washington and NATO voiced concern that China could be preparing to supply Russia with weapons to pursue its war in Ukraine.
"We will not be overwhelmed by threats and pressure from third parties," Wang said.
On Tuesday, Putin announced the suspension of Moscow's participation in the New START arms treaty and vowed to press on with the campaign in Ukraine, during a state of the nation address.
- Wang in Russia -
On Wednesday, Russian lawmakers approved the suspension of the 2010 deal -- the last remaining nuclear arms control treaty between the world's two main nuclear powers.
Putin's treaty announcement was met with widespread international condemnation, though Russia's foreign ministry later said Moscow would continue to comply with the treaty's restrictions in a "responsible approach".
Biden on Tuesday delivered a rock star-style speech to a flag-waving crowd in Warsaw, one day after making a surprise visit to Kyiv.
He vowed that Ukraine would "never be a victory for Russia" and that "our support for Ukraine will not waver".
Russia's state-organised patriotic rally packed Moscow's main Luzhniki stadium for performances and speeches hailing Russian fighters in Ukraine.
"I just heard from the top military leadership of the country that a battle is ongoing right now, for our historical lands, for our people," Putin said.
Wang is on the last stop of a European tour during which he also met Ukraine's Foreign Minister Dmytro Kuleba.
Beijing has sought to position itself as a neutral party, while maintaining close ties with strategic ally Russia.
It has said it is "deeply concerned" and that the conflict is "intensifying and even getting out of control".
China has promised to publish a proposed "political solution" to the Ukraine conflict this week.
- Tensions with Wagner -
At the start of the meeting with Wang, Putin said cooperation between Russia and China was "very important to stabilise the international situation".
When the Kremlin launched its Ukraine offensive, the operation was designed to be a rapid conquest leading to capitulation and the installation of a pro-Russian regime.
Since then, Russia has been forced to give up ground but has kept up a barrage of drone and missile attacks, while the military and civilian toll has spiralled.
Various Western sources estimate the conflict has caused 150,000 casualties on each side.
Moscow's relentless struggle to capture the city of Bakhmut has also exposed tensions between the Russian military and the Wagner mercenary group.
The group's head, Yevgeny Prigozhin, on Wednesday urged Russians to pressure the country's regular army into sharing ammunition with his fighters in Ukraine.
Prigozhin stopped short of calling on Russians to protest but urged everyone from "driver" to "flight attendant" to help him.
He already made headlines on Tuesday when he accused Russia's top brass of essentially committing "treason".
Russia's defence ministry denounced reports of ammunition shortages as "absolutely false".
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