British UN envoy: 'Free and fair referendum cannot take place'
11 March 2014 Last updated at 05:53 GMT
Nato says it will deploy reconnaissance planes over Poland and Romania, as part of efforts to monitor the situation in Ukraine.
The United Nations Security Council has met for the fifth time in 10 days to discuss the crisis.
Britain's ambassador to the UN, Mark Lyall Grant, said that "a free and fair referendum can not take place while Crimea is under the illegal control of Russian troops".