President Obama has warned of "costs" for any military intervention in Ukraine, as reports emerged of military deployments in the Crimea-region.
US officials confirmed tonight that uniformed Russian forces - apparently a hybrid of military and paramilitary - were flying into Simferopol in Crimea.
Mr Obama said that the US was "deeply concerned" over the military movements taken by Russia inside the area. He added that any violation of Ukraine's sovereignty would be deeply destabilising.
It appears as if a full scale 'invasion' is underway, ITV News' Europe Editor James Mates reported from Simferopol:
Russia's UN ambassador Vitaly Churkin however defended the military's presence, saying it was in line with Moscow's existing arrangement with Ukraine on the deployment of military assets in the former Soviet republic.
"We are acting within the framework of that agreement," he told reporters after a closed-door meeting of the UN Security Council.
While Ukraine accused Russia of illegally sending unauthorised military assets across the border. The country's acting President Oleksandr Turchynov appealed to its president Vladimir Putin to stop provocations.
The Defence Ministry later reported that had received information of unknown "radical forces" planning operations against its military units in Crimea early tomorrow morning.
"In the case of such unknown actions, the Ukrainian armed forces will act in accordance with the laws of Ukraine and the regulations of the Ukrainian armed forces," the ministry said in a statement on its website.