But even as Obama warned that Moscow could face further punishments, leaders of Britain, France and Germany were lining up to meet with Russian President Vladimir Putin at week's end.
Those one-on-one meetings would appear to send a mixed message about the West's approach to relations with Russia, given that the same leaders are also boycotting a summit Putin had been scheduled to host this week.
Obama does not plan to hold a formal meeting with Putin while both attend events Friday marking the 70th anniversary of the D-Day invasion that hastened the end of World War II, though the two leaders are likely to have some interaction. The U.S. president suggested there was no contradiction between efforts to isolate Russia and engaging directly with Putin.
"The fact of the matter is that Russia is a significant country with incredibly gifted people, resources, an enormous land mass, and they rightfully play an important role on the world stage and in the region," Obama said during a news conference with Polish President Bronislaw Komorowski. He added that it could be possible for Putin to "rebuild some of the trust that's been shattered during this past year" but said that would take time.
Western leaders, including Obama, have spoken with Putin by phone multiple times since Russia annexed the Crimean Peninsula from Ukraine and stationed tens of thousands of troops on its border with the former Soviet republic. But until this week, they've avoided face-to-face meetings with Putin to avoid giving the impression that the Russian leader can slide back into normal relationships with U.S. and European leaders that have accused him of stoking instability in Ukraine.
Putin's meetings this week will be closely watched by Poland and other Central and Eastern European nations. Many countries in the regions have been pressing for broader NATO assistance to serve as a buffer in case Russia tries to advance beyond Ukraine.
Obama's announcement Tuesday of a "European Reassurance Initiative," costing up to $1 billion, was aimed at quelling some of that anxiety. It marks a significant departure from a two-decade trend toward a smaller U.S. military presence in Europe amid a shift by the Obama administration to a more visible and active naval and air power presence in the Asia-Pacific region. Just three years ago the Pentagon downgraded the top U.S. Army Europe commander from a four-star to a three-star general.
If the U.S. Congress approves the funding, the Pentagon would ramp up its air and ground force rotations in Europe, as well as boost military exercises and position more equipment on the continent.
The plan also calls for increasing the U.S. Navy participation in NATO deployments in the Black and Baltic Seas and helping non-NATO nations such as Georgia, Moldova and Ukraine bolster their own defenses. But key details of the effort were unclear, including how big the U.S. troop increase on the continent might be.
Obama said the fund would be "a powerful demonstration of America's unshakeable commitment to our NATO allies."
Komorowski, appearing with the American president, announced that Poland intended to increase its own defense budget to 2 percent of its gross domestic product, and he urged other NATO nations to do the same.
Obama arrived in Warsaw for the start of a three-country swing through Europe that takes him next to Belgium for a meeting of the Group of 7 major industrial nations, then on to France for the D-Day commemorations.
His itinerary in Poland was filled with stops aimed at highlighting the West's ties to this former communist nation and its neighbors. Moments after Air Force One landed in Warsaw, Obama strode across the tarmac to view four F-16 fighter jets and the American and Polish airmen and soldiers who cooperate on NATO missions. Obama and Komorowski also convened a security meeting and dinner with leaders from Bulgaria, the Czech Republic, Estonia, Croatia, Hungary, Latvia, Lithuania, Romania and Slovakia.
The centerpiece of Obama's visit to Warsaw comes Wednesday, when he'll speak at a ceremony marking the 25th anniversary of Poland's first free election and its emergence from communism. Ukraine's new president-elect, Petro Poroshenko, will also attend the event and hold talks with Obama, underscoring Washington's efforts to underscore the legitimacy of the fledgling government in Kiev.
Putin is expected to dine with French President Francois Hollande Thursday night and hold separate meetings with British Prime Minister David Cameron and German Chancellor Angela Merkel.
All three Western leaders are part of the coalition of wealthy nations that decided earlier this year to boycott the Group of 8 summit Putin had planned to host in Sochi, Russia, the site of this year's Winter Olympics. Instead, they scheduled the upcoming Group of 7 meeting in Brussels, pointedly excluding Putin.
The G-7 leaders are expected to discuss the question of which Russian actions in Ukraine could set off further U.S. and European Union sanctions — either expanding existing penalties or levying deeper penalties against the Russian economy. Unless Russia significantly escalates the situation in Ukraine, it's unclear whether European leaders have any appetite for more sanctions given their broad economic ties to Russia.
Associated Press writers Nedra Pickler and Matthew Lee in Warsaw and Raf Casert in Brussels contributed to this report.
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