Obama touched down in the Polish capital on Tuesday morning to open a European tour likely to be dominated by the unrest in Ukraine. The president's first stop was an inspection of a detachment of US and Polish pilots flying NATO F-16 missions over Poland that were stepped up in response to Russia's annexation of Crimea.

"I've come here, first and foremost, to reaffirm the enduring commitment of the US to the security of Poland. As NATO allies, we have an Article 5 duty to our collective defense," Obama said in a joint press conference with President Bronislaw Komorowski, referring to the NATO charter item that treats an attack on one member country as an attack against them all.

Obama also said that the US would station more troops in the region, saying that he would ask Congress to approve $1 billion (735 million euros) in additional spending to cover the engagement.

"Every ally needs to carry their share and truly invest in the capabilities of the Alliance that are needed for the future," Obama said. "Poland and your president have displayed outstanding leadership in recent months."

Regional talks, Poroshenko meeting

The US president was set to meet a group of NATO leaders from central and eastern Ukraine, including Komorowski and Polish Prime Minister Donald Tusk, in downtown Warsaw for talks on Ukraine. Other countries represented at Warsaw's presidential palace will include Bulgaria, Croatia, the Czech Republic, Estonia, Hungary, Latvia, Lithuania, Romania and Slovakia.

Late on Tuesday, Obama was scheduled to meet Ukraine's president-elect, Petro Poroshenko.

Obama's arrival also coincides with Polish ceremonies marking 25 years since the country's first partly free post-Communist elections, an event with increased significance in light of developments in Poland's neighbor to the east.

The White House has said that Obama will not formally speak one-on-one with Russian President Vladimir Putin at June 6 ceremonies in France on the 70th anniversary of the Allied D-Day landings in Normandy during World War II. An informal discussion, which would be the first in person since the Ukraine tensions flared, has not been ruled out.

Poland is a member of both the European Union and NATO.

msh/dr (AFP, AP)