NATO and Russia have agreed to meet Wednesday for talks on the crisis in Ukraine.
The meeting in Brussels will be the first public contact between the Western defense alliance and Russia's envoys since Russian forces moved into Ukraine's Crimean peninsula late last week.
NATO said Tuesday the Russian military presence in Ukraine presents "serious implications for the security and stability of the Euro-Atlantic area."
Meanwhile in Paris, U.S. Secretary of State John Kerry is expected to meet Wednesday with Russian Foreign Minister Sergei Lavrov.
Kerry was in Kyiv on Tuesday to meet with Ukrainian officials and said the United States wants to see a de-escalation of the crisis.
"It is diplomacy and respect for sovereignty, not unilateral force, that can best solve disputes like this in the 21st century. President Obama and I want to make it clear to Russia and to everybody in the world that we are not seeking confrontation. There's a better way for Russia to pursue its legitimate interests in Ukraine."
At a fundraiser Tuesday night, Mr. Obama said Russia is breaching international law and using troops to "try to force the hands of the Ukrainian people." He told reporters earlier in the day that the international community wants to make sure the rights of Ukrainians are upheld.
"I think everybody recognizes that although Russia has legitimate interests in what happens in a neighboring state, that does not give it the right to use force as a means of exerting influence inside of that state."
He called on Russia to open talks with the interim Ukrainian government, and to allow international monitors to determine whether ethnic Russians in Ukraine are under threat, as alleged by Moscow.
Mr. Obama's comments followed a news conference in Moscow by Russian President Vladimir Putin, who defended his country's military intervention in Crimea.
The Russian leader said he reserves the right to protect Russians in Ukraine. But he also insisted that gunmen blocking Ukrainian military units in the region are "local self-defense forces," not Russian soldiers.
President Obama countered that Moscow has no legal right to intervene militarily, while acknowledging that Mr. Putin "seems to have a different set of lawyers making a different set of interpretations."
The Obama administration also announced a $1 billion energy subsidy package for Ukraine, which is teetering on the brink of bankruptcy.
The Crimean peninsula was placed under Ukrainian control in 1954 by then-Soviet leader Nikita Khrushchev. It remained part of Ukraine when the Soviet Union collapsed in 1991. Crimea has a tiny border with Russia on its far eastern point, and the Crimean port of Sevastapol is home to Russia's Black Sea fleet.
Most residents of Crimea are ethnic Russians, but the region also is home to ethnic Muslim Tatars, who generally show disdain for Russia.
Ukrainian officials say Moscow has sent 16,000 troops into Crimea since last week.
Ukraine's troubles began in November, when president Viktor Yanukovych backed out of a trade deal with the European Union in favor of closer ties and economic aid from Russia. The move triggered weeks of pro-Western anti-government demonstrations in Kyiv and elsewhere in Ukraine, and forced the pro-Russian Yanukovych to flee the capital in late February.