In a clear warning to Ukraine, Putin on Wednesday ordered massive military exercises involving most of the military units in western Russia according to AP press reports.
On Thursday, as part of the exercises, 90 fighter jets were put on combat alert and were patrolling the border with Ukraine, Russian news agencies quoted the Defense Ministry as saying.
The military also announced measures to tighten security at the headquarters of Russia's Black Sea Fleet on the Crimean peninsula in southeastern Ukraine.
The military maneuvers prompted a sharp rebuke from U.S. Secretary of State John Kerry, who warned Russia that any military intervention in Ukraine would be a "grave mistake."