The warning comes after Ukraine blitzed two of Russia's most critical airfields in a devastating drone assault. The audacious strike has been called a 'black day for Russian aviation' - and now defense experts are warning Moscow is 'building up stocks' for a possible offensive against NATO's Baltic state members.

Ukraine’s audacious drone attack on four Russian air bases has prompted speculations about Moscow’s response. The surprise assault ‘embarrassed’ Russia, exposing its vulnerabilities. How will Russian President Vladimir Putin choose to retaliate?

Russia has used its fleet of bombers to rain hellfire on Ukraine. On June 1, Kyiv responded by going after those bombers.