Russia's defence ministry on Wednesday blamed the illegal use of mobile phones by its soldiers for a deadly Ukrainian missile strike that it said killed 89 servicemen, raising the reported death toll significantly.

Moscow previously said 63 Russian soldiers were killed in the weekend strike. The ministry's reaction came amid mounting anger among some Russian commentators, who are increasingly vocal about what they see as a half-hearted campaign in Ukraine.

Most of the anger on social media was directed at military commanders rather than Russian President Vladimir Putin, who has not commented publicly on the attack which was another blow after major battlefield retreats in recent months.

Although an official probe has been launched, the main reason for the attack was clearly the illegal mass use of mobile phones by servicemen, the ministry said.

"This factor allowed the enemy to track and determine the coordinates of the soldiers' location for a missile strike," it said in a statement issued just after 1 a.m. in Moscow on Wednesday (2200 GMT Tuesday).

Ukrainian President Volodymyr Zelenskiy, who rarely comments on specific Ukrainian military strikes, made no mention of the attack in a video address on Tuesday in which he said Russia was set to launch a major offensive to improve its fortunes.