Ukrainian warships in Sevastopol have spent the night draped with mattresses, to slow down any Russian troops attempting to board using grappling hooks.

On Saturday, at a UN Security Council meeting, Moscow's envoy claimed that ousted Ukrainian President Viktor Yanukovych had asked Russia to send troops across the border to protect civilians.

Ukraine's main military bases on the Crimean peninsula remain surrounded by Russian forces.

An alleged ultimatum for Ukrainian forces to surrender - denied by Moscow - expired without incident.