Obama and Merkel spoke by phone Saturday.
The White House says they agreed that Russian intervention, even under purported humanitarian auspices, would "provoke additional consequences."
The leaders also reiterated their commitment to urging Russia to seeking a political solution to the crisis in Ukraine.
Obama spoke to Merkel from Air Force One as he flew to Martha's Vineyard, the Massachusetts island where he is spending his two-week summer vacation.
—By The Associated Press