(Reuters) - Pro-Russian separatists renewed attacks on Ukrainian forces at an airport complex in the east on Monday after Kiev launched a mass operation to reclaim lost ground there that Russia called a "strategic mistake."

Ukrainian officials said three soldiers had been killed and 66 wounded over the past 24 hours, during which they said they had returned battle lines at the airport to the status quo under a much violated international peace plan.

Russia expressed concern at what it called escalation by Kiev and published its own peace plan on Monday in the form of a letter from President Vladimir Putin to Ukraine's President Petro Poroshenko, which it said Poroshenko had rejected.

"It's the biggest, even strategic mistake of the Ukrainian authorities to bank on a military solution to the crisis," Interfax quoted Deputy Foreign Minister Grigory Karasin as saying. "This may lead to irreversible consequences for Ukrainian statehood."