President Barack Obama has announced plans for a $1bn (£600m) fund to increase US military deployments to Europe, during a visit to Poland.
Obama, who will meet Nato leaders amid concerns over the Ukraine crisis, said the security of America's European allies was "sacrosanct".
In April, 150 US soldiers were sent to Poland for military exercises amid growing tensions with Russia.
Obama will also visit Belgium and France during his tour.
Meanwhile, Nato defence ministers are set to meet in Brussels to discuss the long-term security implications of Russian actions over Ukraine.
The Kremlin denies Western claims that it is supporting separatists in eastern Ukraine.
'STEP UP PARTNERSHIP'
"Our commitment to Poland's security as well as the security of our allies in central and eastern Europe is a cornerstone of our own security and it is sacrosanct," Obama said after inspecting a joint unit of US and Polish F-16 pilots.
Speaking at a news conference in Warsaw with Polish President Bronislaw Komorowski, Obama warned Russia against provoking further tensions in Ukraine.
He said Moscow should use its influence to call off separatists in Ukraine's east.
Obama reaffirmed the US commitment to Nato's principle of collective defence, announcing plans for a $1bn European Reassurance Initiative.
The programme, which will fund additional US military rotations to Europe, will need congressional approval.
The US would also "step up partnership" with countries such as Ukraine and Moldova, he added.
Komorowski said Poland would increase its military budget to 2 per cent of its gross domestic product, a move Obama welcomed as a "reminder that every [Nato] ally needs to carry their share" in the alliance.