"The Russian defence ministry is not summoning and does not plan to summon any reservists to the military commissariats," spokesman Igor Konashenkov said in a statement.

In the statement posted on messaging app Telegram, Konashenkov said that "many" Russian men had in recent days received "false" phone calls notifying them of their summons to the military commissariats.

"All such fake calls are made from Ukrainian territory," Konashenkov said. "They absolutely do not correspond to reality and are a provocation of the Ukrainian special services."

While Ukrainian forces have stalled the initial Russian advance and launched some successful counterattacks, there are signs that both sides are digging in for a long conflict that neither can easily win.

On Friday, Russia signalled it may dial back its aims in Ukraine to focus on the east after failing to break the nation's resistance in a month of fighting.

Russian President Vladimir Putin sent troops to Ukraine on February 24, prompting the West to introduce unprecedented sanctions against Moscow.

The Kremlin's military campaign - and fears of a general mobilisation -- have sparked an exodus, with tens of thousands of Russians leaving the country.

Among those who have left are numerous IT specialists, and the government is putting together proposals aimed at convincing them to remain in the country including a reprieve from compulsory military service.