Obama, meeting Irish Prime Minister Enda Kenny in the Oval Office, reiterated to reporters that there would be consequences should Russia refuse to loosen its grip on the Crimea region of southern Russia.
Crimea's pro-Russia parliament has scheduled a referendum on Sunday to decide whether the region should be annexed by Russia.
"We continue to hope that there is a diplomatic solution to be found," Obama said.
"But the United States and Europe stands united, not only in its message about the Ukrainian sovereignty but also that there will be consequences if, in fact, that sovereignty continues to be violated," he said.
Obama's comments came as his secretary of state, John Kerry, met in London with Russian Foreign Minister Sergei Lavrov. Speaking after several hours of talks, Lavrov said there was still no common vision with the West over Ukraine but Russia has no plans to invade southeastern Ukraine.