"Secretary Tillerson phoned Ukrainian President Petro Poroshenko today to discuss his recent trip to Moscow and his message to the Russian leadership that, although the United States is interested in improving relations with Russia, Russia's actions in eastern Ukraine remain an obstacle," said acting spokesman Mark Toner.
The phone call came as the two leaders discussed the death Sunday of an American monitor with the Organization for Security and Co-operation in Europe, who hit a landmine while on patrol in eastern Ukraine.
The statement said Tillerson accepted Poroshenko's "condolences" for the death of the American.
It marked the first loss for the security body's Special Monitoring Mission (SMM) since Europe's only current war broke out more than three years ago.
Russia has provided support to rebels in war-torn eastern Ukraine, where more than 10,000 people have died in a conflict which this month entered its fourth year.
Toner said Tillerson reiterated Washington's "firm commitment to Ukraine's sovereignty and territorial integrity and confirmed that sanctions will remain in place until Russia returns control of the Crimean peninsula to Ukraine and fully implements its commitments in the Minsk agreements."
He said Tillerson also "emphasized the importance of Ukraine's continued progress on reform and combating corruption."
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