(Adds Kerry comments)
WASHINGTON, March 16 (Reuters) - The United States told Russia on Sunday that it would not accept the results of Crimea's referendum on seceding from Ukraine and it continued to urge a political resolution on Moscow, a senior U.S. State Department official said.
The official, describing a telephone conversation between U.S. Secretary of State John Kerry and Russian Foreign Minister Sergei Lavrov on Sunday morning, the day of the Crimean vote, urged Russia to back constitutional reform in Ukraine that would protect the rights of minorities such as Crimea's Russian-speaking population.
Kerry also said Russia must pull back its forces to their bases and raised strong concerns about Russian military activities in Ukraine's Kherson oblast on Saturday, as well as continuing provocations in eastern Ukraine, the official said.
Kerry and Lavrov have communicated frequently about the Crimea situation, including six hours of talks in London on Friday.
In their Sunday phone conversation, Kerry appeared to be trying to sketch out a way forward for Crimea that would keep it in Ukraine, but it remained unclear if Russian President Vladimir Putin was interested in such an outcome or simply wants to cement Russian control of the region.
Moscow has a naval base in Crimea and in the past two weeks has sharply increased its military presence in the region, which has a Russian-speaking majority. (Reporting By Arshad Mohammed; Editing by Bill Trott and Rosalind Russell)