The secretary of Ukraine's security council, Andriy Paruby, told reporters in Kyiv on Sunday that the body, which is made up of the country's top security and military officers, had issued the call-up to "all those that armed forces need at the moment across Ukraine."
He added that the mobilization was necessary to "ensure the security and territorial integrity of Ukraine."
"This is not a threat: this is actually the declaration of war to my country," acting Prime Minister Arseniy Yatsenyuk told reporters in Kyiv.
Russian approval of troops in Ukraine
The Ukrainian armed forces had already been placed on a state of high alert on Sunday, a day after Russia's upper house of parliament approved a request from President Vladimir Putin to authorize the use of the country's military in Ukraine. Parliamentarians approved the use of Russian troops within the territory of its much smaller neighbor "until the social and political situation in that country is normalized."
In his request, Putin had argued that sending in troops might be necessary to counter a threat to Russian citizens, ethnic Russians and Russian military personnel already in the Crimea.
This was met with condemnation from the United States and its European Union allies.
90-minute telephone call
US President Barack Obama used a telephone conversation with Putin on Saturday to express his displeasure at the move. During their conversation, which lasted a full 90 minutes, Obama reportedly told Putin that Russia had violated international law by getting involved in Crimea.
A statement released by the Kremlin, though, said that "if violence spread further in the eastern regions of Ukraine and in Crimea, Russia reserves the right to protect its interests and those of Russian speakers living there."
Just hours before ambassadors from the Western military alliance NATO met in Brussels to discuss the crisis, there were reports of more military activity in the Crimea region.
The Associated Press reported that its journalists had seen hundreds of military trucks carrying troops and a Tiger vehicle armed with a machine gun traveling along the road from Sevastopol, where Russia's Black Sea Fleet is located, to Simferopol on Sunday morning.
Russia's Interfax news agency, meanwhile, cited a Ukrainian Defense Ministry source who said Russian troops had seized weapons from a radar base near the town of Sudak as well as a training center in Sevastopol.
In other diplomatic developments, British Foreign Secretary William Hague was to arrvive in Kyiv to discuss the crisis later on Sunday. Canada, meanwhile, has recalled its ambassador to Russia "for consultations," according to a statement released by Prime Minister Stephen Harper following an emergency cabinet meeting on Saturday.
pfd/tj (dpa, Reuters, AP, AFP)