Russia's defense ministry said on Monday 63 of its soldiers were killed in a Ukrainian rocket strike in the eastern region of Donbas. It also said Ukraine used a HIMARS rocket launcher supplied by the US.

Ukrainian media and Russian independent media reported that the strike took place on Saturday night. They also said those killed were mobilized reservists, and that the death toll may be actually higher, possibly in the hundreds.

Russia's state-run Tass news agency cited local officials saying that frequent use of mobile phones by the soldiers at the facility could have drawn the attack. It explained that data on their location could have been captured by Ukrainian forces.

Observers say it is unusual for Moscow to acknowledge such a large number of casualties, pointing out the strike could have a huge impact on Russia's military forces.

Also on Monday, the Ukrainian military's commander-in-chief Valerii Zaluzhnyi said Ukraine has recaptured 40 percent of areas where Russia had taken control since it began the invasion February last year. He also said that Ukrainian forces have killed 100,000 Russian troops, and removed explosives from 1,346 hectares of areas.

Zaluzhnyi stressed that Ukrainian forces are fighting not only for their country, but also to maintain peace in Europe and rest of the world.