Defence Secretary Michael Fallon said that Nato was taking the threat posed by Russia seriously.
"What's happened in the Ukraine is very much a wake-up call which the Nato leaders will be discussing at their summit in a month's time here in the UK," he said.
Mr Fallon said that Britain was contributing Typhoon fighter jets to the Baltic policing mission along with other Nato members.
An RAF surveillance aircraft was also in the sky over Ukraine's eastern border with Russia, he added.
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