President Barack Obama has announced plans for a $1bn (£600m) fund to increase US military deployments to Europe, during a visit to Poland.
Mr 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 Moscow.
Mr 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.
'Step up partnership'
Speaking at a joint press conference in Warsaw with Polish President Bronislaw Komorowski, Mr Obama warned Russia against provoking further tensions in Ukraine.
He said Moscow should use its influence to call off separatists in Ukraine's east.
Mr 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.
Mr Komorowski said Poland would increase its military budget to 2% of its gross domestic product, a move Mr Obama welcomed as a "reminder that every [Nato] ally needs to carry their share" in the alliance.
Earlier on Tuesday, Mr Obama met US and Polish air personnel from a small detachment of F-16 fighter jets based in Poland.
'Feeling neglected'
In a statement, the White House said the European Reassurance Initiative would not "come at the expense of other defence priorities, such as our commitment to the Asia Pacific rebalance".
President Obama: ''''We will be expanding our exercises and training with allies to increase the readiness of our forces''
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