The Russian military has given Ukrainian forces in Crimea until 5 a.m. Tuesday (10 p.m. ET Monday) to surrender or face a "storm," Interfax news agency reported.
"If they do not surrender by 5 a.m. tomorrow, we will start a real storm in Ukrainian bases in Crimea," according to the statement sent by the Russians to the Ukrainian Defense Ministry, a ministry source told Interfax. NBC News could not immediately confirm the report.
The ultimatum was attributed to Alexandr Vitko, chief commander of Russia's Black Sea fleet.
Ukraine mobilized for war on Sunday after Russian President Vladimir Putin declared he had the right to invade his neighbor to protect Russian citizens.
The standoff in Ukraine has created the greatest moment of tension between Russia and the West since the collapse of the Soviet Union in 1991, an event Russian President Vladimir Putin once called the worst geopolitical catastrophe of the 20th century.
Reuters contributed to this report.