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Ukraine is not a NATO member but Russia's intervention in Crimea has alarmed neighbouring countries, including alliance members that used to be dominated by the Soviet Union.
Acting on a recommendation from the alliance's top military commander, U.S. Air Force General Philip Breedlove, NATO ambassadors gave the go-ahead to the AWACS flights, a NATO spokesman said.
AWACS (airborne early warning and control) planes will fly from their home airbases in Geilenkirchen, Germany, and Waddington in Britain, the spokesman said.
"These flights will enhance the alliance's situational awareness and all will take place solely over alliance territory," he said.
The flights will start soon and go on for as long as required, he added.
The Western military alliance announced a review of its cooperation with Moscow last week after Russian forces tightened their grip on Crimea, home to the Russian Black Sea Fleet, following weeks of upheaval in Ukraine which culminated in the ousting of President Viktor Yanukovich.
NATO allies met last Tuesday after Poland invoked a rule allowing any ally to consult with the others if it feels its security is under threat.
Soon after that meeting, U.S. officials said the Pentagon would more than double the number of U.S. fighter jets on a NATO air patrol mission in the Baltics and do more training with Poland's air force in an attempt to reassure allies.