Mr. Obama will meet NATO leaders amid concerns over the Ukraine crisis, said the security of America’s European allies was “sacrosanct.”
In April, there were 150 U.S. soldiers sent to Poland for military exercises were soaring tensions with Russia.
Both Belgium and France will be visited by Mr. Obama during his tour.
NATO defense ministers are set to meet in Brussels to discuss the long-term security implications of Russian actions over Ukraine.
The Kremlin denies allegations of Western claims that it’s 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,” Mr. Obama said after inspecting a joint unit of U.S. and Polish F-16 pilots.
While speaking at a news 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 U.S. commitment to NATO’s principle of collective defense, announcing plans for a $1 billion European Reassurance Initiative.
The program is intended to fund additional U.S. military rotations to Europe, which will need congressional approval.
The U.S. would also “step up partnership” with countries like 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.
NATO defense spending targets
NATO’s members have a target of spending 2% of their GDP on defense
Countries that met or exceeded the target in 2013 included:
- United States – 4.4%
- United Kingdom – 2.4%
- Greece – 2.3%
- Estonia – 2%
Countries that spent less than the target in 2013 included;
- France – 1.9%
- Turkey – 1.8%
- Poland – 1.8%
- Germany – 1.3%
- Lithuania – 0.8%
Overall, European members of NATO spent 1.6%
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.”
Mr. Obama’s emphasis on relations with Asian nations has left some Eastern European leaders feeling neglected in recent years, the BBC’s Adam Easton in Warsaw reports.
In Belgium, Mr. Obama is expected to urge Western leaders to reaffirm their united position on Ukraine at a G7 meeting of major industrial nations.
The summit should be held in Russia. However, Western leaders decided to change its venue because of Moscow's annexation of Crimea in March.
The US leader will take part, in France, to commemorate the 70th anniversary of the D-Day landings in Normandy.
President Vladimir Putin is invited to attend the summit, but the White House has already made it clear that both American and Russian leaders will not hold formal bilateral talks.
Washington and its European allies urged Moscow to de-escalate tensions in eastern Ukraine, where fighting has continued between separatists and government troops.
On Monday, hundreds of rebels attacked a border command center near the eastern city of Luhansk.