resolve after the Russian intervention in Ukraine.
The White House unveiled plans for a $1 billion initiative to send more of its military to Europe on a temporary basis but stopped short of promising to beef up its permanent presence as some of Washington's allies are seeking. It said the US would review its force presence on the continent.
Speaking in an aircraft hangar at Warsaw airport where he met US airmen taking part in a joint programme with the Polish air force, Mr Obama said US commitments to Poland and the region were a cornerstone of the United States' own security.
"As friends and allies we stand united together," said Mr Obama, whose two-day stay in Warsaw will include meetings with Ukrainian President-elect Petro Poroshenko and other central and eastern European leaders.
Mr Obama is under pressure from critics at home, who say he is not showing enough firm leadership on the world stage, and from some Nato allies in eastern Europe who fear they may be the next targets for Russian expansion and want more US protection.
But Western powers must also strike a delicate balance, because a big increase in Nato forces on Russia's borders could prompt reciprocal steps from the Kremlin and spiral into an arms race.
Nato defence ministers met in Brussels yesterday to look at long term measures to strengthen alliance defences in eastern Europe.