The White House said both leaders were worried about "Russia's materiel support for the separatists" and its failure to uphold a ceasefire agreement signed last September in Minsk.
It added in a statement that they "agreed on the need to hold Russia accountable for its actions".
Washington is ready to ramp up sanctions on Russia over Ukraine if necessary, US Treasury Secretary Jack Lew said at the signature of a USD 2 billion loan guarantee agreement for Kiev on Wednesday. Obama and Merkel spoke of the need for a "robust package of financial support" to help stabilise Ukraine.
European Union foreign ministers are likely to ask the bloc's executive Commission to prepare new sanctions against Russia at a meeting on Thursday, though a final decision will be taken by EU leaders at a summit on Februay 12.

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