Under fire from critics at home who say his leadership on the world stage has not been muscular enough, Obama unveiled plans to spend up to $1 billion in supporting and training the armed forces of NATO states on Russia's borders.
The White House also said it would review permanent troop deployments in Europe in the light of the Ukraine crisis -- though that fell short of a firm commitment to put troops on the ground that Poland and some of its neighbors had sought.
Stationing troops permanently in eastern Europe would be tricky: many NATO members in Western Europe would baulk at the cost, and a big increase in U.S. forces could prompt reciprocal steps by Moscow and spiral into an arms race.
"We need to make sure that the collective defense ... is robust, it is ready, it is properly equipped," Obama told a joint news conference with Polish President Bronislaw Komorowski in Warsaw at the start of a four-day visit to Europe.
"The United States is proud to bear its share of the defense of the transatlantic alliance," he said after their talks in Warsaw. "It is the cornerstone of our security."
But he also said other NATO states - many of which lag far behind the alliance's target of spending 2 percent of gross domestic product on defense - needed to do their share as well. "We can't do it alone," the U.S. leader said.
As they met, fighting raged in eastern Ukraine for a second straight day as Kiev's army pressed an offensive against pro-Russian separatists holding the city of Slaviansk and said it had inflicted losses on the rebels.
Obama was to meet Ukraine's President-elect Petro Poroshenko in Warsaw later on Wednesday and will attend celebrations in France with Russian President Vladimir Putin on Friday to mark the 70th anniversary of the World War Two D-Day landings.
The Kremlin said Putin would hold private meetings on the sidelines with German Chancellor Angela Merkel, French President Francois Hollande and British Prime Minister David Cameron, but the Russian leader had no plans to meet Obama.
The U.S. leader said he had no interest in threatening Russia, but that it must respect Ukraine's sovereignty, rein in separatist fighters there, and work together with Poroshenko. If Russia did not, Obama said, more sanctions have been prepared.
"Mr Putin has a choice to make," Obama said. "That's what I will tell him if I see him publicly. Thatâs what I have told him privately."
Obama said he would offer Poroshenko U.S. support for the Ukrainian economy to help ensure it can get through the winter if Moscow turned off gas supplies in a row over payment.
"I want to hear from him (Poroshenko) what he thinks would be most helpful," Obama said. "Weâre going to spend a lot of time on the economics of Ukraine."
Washington recognized that Russia had a historic relationship with Ukraine and had legitimate interests in what happened along its border, he said.
"But we also believe that the principles of territorial integrity and sovereignty have to be respected," he said. "We have prepared economic costs on Russia that can escalate if in fact we continue to see Russia actively destabilizing one of its neighbors in the way that weâve seen of late."
Poland, which spent much of its history under Russian domination and is now one of the most hawkish NATO members, has previously said it wanted a large U.S. force on its soil as soon as possible.
However, Komorowski said the U.S. pledge on military support was a good response to the security threats in the region since Russia annexed Ukraine's Crimea peninsula earlier this year.
"For us it is most important that it is made clear that there are no second-rate NATO members. That there are no countries that are told by some outside countries, particularly Russia, whether U.S. forces can or cannot be stationed there," Komorowski said.