KIEV, Ukraine — Russian President Vladimir Putin on Sunday defended the separatist drive in the disputed Crimean Peninsula as in keeping with international law, but Ukraine’s prime minister vowed not to relinquish “a single centimeter” of his country’s territory.

Over the weekend, the Kremlin beefed up its military presence in Crimea, a part of Ukraine since 1954, and pro-Russia forces keep pushing for a vote in favor of reunification with Moscow in a referendum scheduled for next Sunday.

President Barack Obama has warned that the March 16 vote would violate international law. But Putin made it clear that he supports the referendum in phone calls with German Chancellor Angela Merkel and British Minister David Cameron.

Ukraine Prime Minister Arseniy Yatsenyuk said he will meet with Obama in Washington on Wednesday.