Ammunition and weapon storage bunkers were destroyed at Russia's Engels airbase, according to new satellite imagery. The airbase was targeted by Ukrainian drones last week on Thursday. A massive fireball and mushroom cloud were seen above the airfield after the attack.

Most of the damage occurred at storage areas about a mile east of the runway, with at least a dozen storage facilities and other nearby buildings were destroyed. The extensive damage was caused by huge secondary explosions when the drones detonated explosive ordnance stored there.

Several Ukrainian accounts claimed that three Tu-95MS Bear-H and two Tu-160 Blackjack strategic bombers were parked at the airbase when the UAVs struck.

Russian Telegram channels said that one of the Bear bombers was damaged by shrapnel and that two pilots were killed.

However, there has been no official confirmation of casualties from either the Russians or Ukrainians.

Meanwhile, Russia and Ukraine continued to attack each other overnight on Monday.

Ukraine’s Air Force said it had shot down 57 of 99 drones launched overnight, while Russia reported 28 drones were intercepted in the other direction.

Ukraine's railway also reported that its online systems had been hit by a large-scale cyber-attack.

Ukrzaliznytsia said it first reported disruption to its systems on Sunday, calling it an IT failure.

"The railway continues to move despite physical attacks on the infrastructure, not stopping it even with the most vile cyberattacks!" The company said on Telegram, adding that train traffic is stable and running without delays.

It comes as US and Russian peace negotiators prepare to hold further talks on Monday in Saudi Arabia.

Reports say that the Russian delegation will include career diplomat Grigory Karasin and Sergei Beseda - an adviser to the head of the FSB spy agency.

The summit follows a meeting behind closed doors on Sunday between US and Ukrainian officials in the Saudi capital.

The Ukrainian delegation was led by Defence Minister Rustem Umerov, who described the talks as "productive".