Barack 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.
Barack Obama will also visit Belgium and France during his tour.
Meanwhile, NATO defense 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.
“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,” Barack 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, Barack Obama warned Russia against provoking further tensions in Ukraine.
He said Moscow should use its influence to call off separatists in Ukraine’s east.
Barack Obama reaffirmed the US commitment to NATO’s principle of collective defense, announcing plans for a $1 billion European Reassurance Initiative.
The program, 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.
Bronislaw Komorowski said Poland would increase its military budget to 2% of its GDP, a move Barack Obama welcomed as a “reminder that every [NATO] ally needs to carry their share” in the alliance.
In a statement, the White House said the European Reassurance Initiative would not “come at the expense of other defense priorities, such as our commitment to the Asia Pacific rebalance”.
In France, Barack will take part in ceremonies to commemorate the 70th anniversary of the D-Day landings in Normandy.
Russian President Vladimir Putin is also invited, but the White House has already made it clear that the American and Russian leaders will not hold formal bilateral talks.
Washington and its European allies have repeatedly urged Moscow to de-escalate tensions in eastern Ukraine, where fighting has continued between separatists and government troops.
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