Even with strong cultural and commercial ties between Russia and Ukraine, "what cannot be done is for Russia with impunity to put its soldiers on the ground and violate basic principles that are recognized around the world," Obama said during a media availability with Israeli Prime Minister Binyamin Netanyahu. "And I think the strong condemnation that has proceeded from countries around the world indicates the degree to which Russia is on the wrong side of history."
Russian leaders maintain it is their right and obligation to protect Russian citizens and pro-Russian people in Ukraine where Moscow ally Viktor Yanukovych was ousted as president two weeks ago, fled Kiev and took refuge in Russia. There have been no verifiable reports of violence against Russians in Ukraine.
If Russia continues down that path, "we are examining a whole series of steps -- economic, diplomatic -- that will isolate Russia and will have a negative impact on Russia's economy and status in the world," Obama said.
"We've already suspended preparations for the [Group of Eight] summit, and we would expect there would be further follow-up on that," he said.
"Obviously the facts on the ground in Crimea are deeply troubling, and Russia has a large Army that borders Ukraine," Obama said. "But what is also true is that over time this will be a costly proposition for Russia, and now is the time for them to consider whether they can serve their interests in a way that resorts to diplomacy as opposed to force."
He said Congress should work with his administration to help provide an aid package to Ukrainians.
"[There] should be unanimity among Democrats and Republicans that when it comes to preserving the principle that no country has the right to send in troops to another country unprovoked," Obama said. "And my expectation is, is that I'll be able to get Congress to work with us in order to achieve that goal."
The United States is considering actions beyond sanctions against Russia over its incursion into Ukraine, a U.S. State Department spokeswoman Jan Psaki said Monday.
Psaki said she didn't have any independent information on an Interfax report quoting a Ukrainian Defense Ministry official that the Russian fleet commander gave an ultimatum to Ukrainian forces in Crimea to surrender or face attack. Moscow has denied issuing such an ultimatum, Russian media reported.
"[These] reports today of threats of force against Ukrainian military installations would, if true, in our view, constitute a dangerous escalation of the situation for which we would hold Russia directly responsible," Psaki said.
In Moscow, meanwhile, the Russian Foreign Ministry lashed out at comments concerning sanctions made by the U.S. Secretary of State John Kerry over possible Russian military action in Ukraine as "unacceptable threats," RIA Novosti reported.
Kerry said the United States was prepared to strip Moscow of its seat with the Group of Eight, withdraw investments and businesses from Russia, impose travel bans and freeze assets.
Russia called on the United States and its allies to stop encouraging what Moscow says are nationalist and anti-Russian initiatives by its interim leadership, and focus on a dialogue and decisions acceptable to both the pro-European west and pro-Russian east.
In Simferopol, the capital of the autonomous Crimea, Kiev Post reported the Russian navy maintained its blockade of Ukrainian naval ships docked at Sevastopol Bay and Ukrainian military posts, barring relatives or others from bringing food to Ukrainian soldiers.
On Sunday Russian President Vladimir Putin agreed to a German proposal for international observers to review the standoff in Ukraine's Crimea area, a spokesman for German Chancellor Angela Merkel said.
The spokesman said Merkel offered the proposal for a "contact group" of mediating foreign diplomats and a delegation of observers to evaluate Moscow's claims ethnic Russians were being threatened under Ukraine's new leadership during a phone call Sunday to Putin, the Los Angeles Times reported.
RIA Novosti, quoting the Ukrainian UNIAN news agency, reported Monday Ukrainian officials have begun criminal proceedings against Yanukovych for attempting to violate the constitution.
The country's interim leaders last week placed Yanukovych on an international wanted list for mass murder charges stemming from clashes between police and anti-government protesters in Kiev that left more than 80 dead days before his ouster.
"After his appearance in Rostov-on-Don [in Russia], it was decided to begin criminal proceedings for attempts to overthrow the constitutional order in Ukraine," UNIAN quoted Prosecutor General Oleg Makhnitsky as saying.
U.S. Secretary of State John Kerry is to go to Kiev Tuesday to meet with the country's interim leaders.
British Foreign Secretary William Hague, in Kiev Monday, warned the standoff over Crimea was "the biggest crisis in Europe in the 21st century" and the international community "cannot just allow this to happen."