When meeting Irish Prime Minister Enda Kenny at the White House, Obama told reporters that there would be "consequences" if Ukraine's sovereignty is violated, Xinhua reported.
"We continued to hope that there's a diplomatic solution to be found, but the United States and Europe stand united not only in its message about Ukrainian sovereignty, but also that there will be consequences if, in fact, that sovereignty continues to be violated," Obama said.
Obama's remarks came as US Secretary of State John Kerry was holding talks with Russian Foreign Minister Sergei Lavrov in London in hopes of finding a diplomatic solution ahead of Sunday's referendum in Ukraine's Crimea region.
Russia will "respect the will of the people of Crimea", Lavrov said after the talks, according to a media report.
Lavrov said that Russia and the US did not have a "common vision" on the crisis in Ukraine, but described his talks with Kerry as "constructive".
On Thursday, Kerry threatened "a very serious series of steps" against Russia with the European Union in response to Crimea's vote to break away from Ukraine.
"If there is no sign of any capacity to be able to move forward and resolve this issue, there will be a very serious series of steps on Monday in Europe and here with respect to the options that are available to us," Kerry said at a Senate panel hearing.