"We shared our concerns over the crisis in Ukraine and agreed on how important it is for Russia to withdraw from eastern Ukraine, to stop supplying troops and equipment, to allow effective control of the border," former Polish premier Tusk said in a statement released by Brussels.
"The European Union and the United States continue working together closely, including on sanctions as well as on financial support to Ukraine. We agreed to stay united and to keep a steady course."
Tusk said the fact that the call with Obama came on the day that he took over as head of the council of the EU's 28 national leaders "underlines the importance I attach to our relations with the US."
The comments were the first by Tusk on the crisis in Ukraine in his new job, having avoided any direct reference to it in a speech during a handover ceremony with his predecessor Herman Van Rompuy earlier Monday.
Tusk added that he and Obama also discussed a mammoth but controversial US-EU trade deal, saying it was an "expression of our geopolitical partnership" and adding that they had "agreed to step up our efforts towards reaching agreement.