Vladimir Putin has scrambled to save face by casting fresh doubt on ISIS claiming responsibility for the Moscow concert massacre that killed nearly 140 people, the deadliest attack on the nation's soil in years.

Two of the four ISIS terrorists who were charged overnight with terrorism have pleaded guilty, after they were caught, allegedly tortured and detained by Russian security services following the deadly attack at a Moscow concert hall on Friday night. One of the terrorists was seen staking out the venue just a few days before the attack.

ISIS twice claimed responsibility for the attack over the weekend, first posting a selfie taken by the four men, all of whom are Russian citizens, in front of an ISIS flag before posting a 90-second selfie video of the devastating attack from the terrorists' perspective.