Ukraine’s interim leaders called on the world to help Sunday, saying the country was “on the brink of disaster.”
Acting Prime Minister PM Arseniy Yatsenyuk said the move by Russian forces to surround military bases in Crimea was "a declaration of war."
His comments were echoed by acting president Oleksandr Turchynov, who said Ukraine had closed its airspace to non-civilian aircraft.
“Ukrainian forces are acting in accordance with Ukrainian laws and Constitution and they will defend Ukrainian territory,” Turchynov told reporters at a news conference in Kiev.
“Any attempt to attack military bases is in fact a direct aggression against our country responsibility for which will be borne by Russian military officers and Russian Government,” he added.
Earlier Sunday, Ukraine's national security council secretary ordered the call-up of military reservists, saying the country's forces must be "combat ready."
Marc Smith and Sasha Mazikina contributed to this report.