In a clear indication of growing US concern, Obama broke away from events in Ohio, where he has been addressing economic hardships facing Americans, to call Merkel about a crisis that threatens to spread across the eurozone.
"President Obama and German Chancellor Angela Merkel spoke today by telephone, one in an ongoing series of consultations between close allies on global issues," the White House said in a statement.
"They discussed the importance of resolute action by Greece and timely support from the International Monetary Fund (IMF) and Europe to address Greece's economic difficulties."
Merkel's government has for weeks shown reluctance to give Greece financial aid because of fierce domestic opposition.
But as the euro was pummeled in recent days and Greek debt was downgraded to junk status, fears that the crisis was spreading appeared to have forced Merkel to act.
The news came as concerns also grew about the ballooning size of the bailout package experts forecast Greece would actually need. Economy Minister Rainer Brüderle suggested Greece’s government would need €135 billion by 2012, with Germany having to kick in €25 billion to €30 billion of that.
His remarks followed suggestions by IMF chief Dominique Strauss-Kahn that Greece would need €120 billion.
However, Finance Minister Wolfgang Schäuble quickly slapped down speculation about a total.
“No, no one knows (what the total will be),” Schäuble told broadcaster ARD. “We hope that it will cost nothing at all,” he added, pointing out that the bail-out would come in the form of loans.
On the margins of crisis meetings with the IMF and the European Central Bank, Merkel on Wednesday said that Greek rescue talks must be "accelerated."
Her comments came as the euro hit fresh one-year lows and Spain became the latest eurozone member to suffer a downgrade of its credit rating.
Greece meanwhile acted to stop speculators operating on the Athens stock exchange as the interest rate it has to pay to borrow money hit 11.1 percent, trailing only Pakistan and Venezuela as the world's highest interest payers.
US firms are not seen as hugely exposed to Greece's debt woes, but instability in the eurozone has rocked US markets, causing the blue-chip Dow to lose over 200 points on Tuesday.
The European Union is also the biggest trading partner for the United States.
According to the White House, "the president and the chancellor also discussed ongoing efforts to support peace in the Middle East."