WASHINGTON — US Secretary of State John Kerry urged Moscow Sunday to pull back Russian forces to their bases in Crimea in return for constitutional reforms in Ukraine to protect minority rights, a US official said.

“The secretary made clear that this crisis can only be resolved politically and that as Ukrainians take the necessary political measures going forward, Russia must reciprocate by pulling forces back to base, and addressing the tensions and concerns about military engagement,” the senior State Department official said.

Kerry made the overture in a telephone call with Russian Foreign Minister Sergei Lavrov Sunday as residents of Crimea were voting in a referendum on whether to break away from Ukraine and join Russia.

Kerry reiterated that the United States considers the referendum illegal under Ukrainian law and that it would not recognize the outcome.

He also raised concerns about Russian military activity in Kherson Oblast, the Ukrainian province just north of Crimea, and “continuing provocations” in the eastern cities in Ukraine.

“Further to discussions that the United States has had with Russia, our European partners and the government of Ukraine, Secretary Kerry also drew attention to the broad multi-party constitutional reform process already underway in the Ukrainian Rada,” said the official, referring to the parliament.

“He urged Russia to support efforts by Ukrainians across the spectrum to address power sharing and decentralization through a constitutional reform process that is broadly inclusive and protects the rights of minorities.”