The military alliance said two AWACS, or Airborne Warning And Control System, reconnaissance planes took off from bases in Germany and Britain.
The surveillance flights won't leave the airspace of its member nations, thus not crossing either into Ukrainian or Russian airspace, a spokesman for NATO's operational headquarters said in Belgium.
"The planes can observe over 300,000 square kilometres and will primarily be looking on air activity and the sea," Lt Col Jay Janzen said, adding that one AWACS aircraft already went on a surveillance mission to Romania yesterday and that more missions were being planned.
NATO's 28 member states decided Monday to intensify the assessment of the possible threat the Ukrainian crisis poses to the alliance by sending AWACS planes. The decision comes after deployments of US fighter planes to eastern European nations bordering Russia, such as Poland and Lithuania.
Janzen said the flights had already been planned as training missions before NATO's decision, but more planes will be added to the exercises in the coming days.
The plane flying out of the German base to Romania was an E-3A AWACS and the plane leaving Britain for Poland was an E-3B AWACS, Janzen said.