Nato is to dispatch Awacs [Airborne Warning and Control System] reconnaissance aircraft to Poland and Romania in order to monitor the Ukraine border after authorisation was given by alliance ambassadors.
Nato diplomats decided on the move after a recommendation from U.S. Air Force Gen. Philip Breedlove, the alliance's top military chief, according to a Nato spokesman.
The reconnaissance aircraft will fly from Nato Air Base Geilenkirchen in Germany and RAF Waddington military base in the United Kingdom. The countries within Nato who are supplying the aircraft are currently unknown.
"All Awacs reconnaissance flights will take place solely over alliance territory," said the official.
The surveillance flights were authorised with the aim of enhancing "the alliance's situational awareness", the spokesman added.
Awacs planes will provide reassurance to Eastern European nations that the alliance is committed to monitoring the crisis and comes a week after the announcement that the United States is to send 12 F-16 fighter jets and 300 service personnel to Poland for a training exercise this week.
The move is "an appropriate and responsible action in line with Nato's decision to intensify our ongoing assessment of the implications of this crisis for Alliance security," the spokesman said of the deployment of reconnaissance aircraft to the region.
The Crimean parliament has voted to join Russia and will put the case to voters in a referendum on 16 March. Crimeans will be asked to vote whether the republic should join Russia or remain part of Ukraine.
A Crimean Tatar leader has warned that occupying Russian forces will face bloody reprisals from militants in the Crimean Peninsula if the Ukrainian region joins Russia in a referendum, while a group of Serbian war veterans identifying themselves as Chetniks have joined pro-Russian troops occupying Ukraine's Crimea region.