In a telephone call about the unfolding crisis, Obama and Merkel "agreed that it was critically important to make every effort to return to a path that will allow Greece to resume reforms and growth within the eurozone," the White House said.
Obama and Merkel said their teams of economic advisors were closely monitoring the rapidly changing situation, the statement said.
Months-long bailout talks between Athens and its creditors dramatically broke down Saturday over the economic reforms demanded by creditors in exchange for more cash needed to keep Greece afloat.
Greek Prime Minister Alexis Tsipras stunned Europe late Friday with a surprise call for a July 5 referendum on the latest cash-for-reforms package and advised voters against backing a deal that he said spelled further "humiliation."
The eurozone responded by refusing to extend the European Union's funding program beyond Tuesday, almost certainly meaning Greece is headed for a default.
And many fear a "no" vote in the referendum would send Greece out of the eurozone.
Ahead of the referendum, US Treasury Secretary Jacob Lew called on all parties to "continue to work to reach a solution, including a discussion of potential debt relief for Greece," a Treasury statement said Sunday.
Lew said it was "in the best interests of Greece, Europe and the global economy for Greece and its creditors to find a sustainable solution," adding Athens would need to adopt "difficult measures."
He also highlighted "the importance of taking necessary steps to maintain financial stability in the run-up to the referendum," the statement said.
For now, the European Central Bank has pledged to maintain emergency liquidity assistance to Greece, offering Athens a bit of a lifeline.
Tsipras announced Sunday that the country's banks and stock market would be closed on Monday, and capital controls imposed, calling for calm.