Pro-Russian separatists have renewed attacks on Ukrainian forces at an airport complex in the east, 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 in 24 hours.

They said they had returned battle lines at the airport outside Donetsk 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 today in the form of a letter from President Vladimir Putin to Ukraine's President Petro Poroshenko.

It said that Mr Poroshenko had rejected the plan.

"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."

In Kiev, Ukrainian Prime Minister Arseny Yatsenyuk said Ukrainian intelligence had confirmed Russian cross-border arms deliveries to the separatists were continuing.

At a joint news conference with Polish Prime Minister EwaKopacz, Mr Yatseniuk listed Russian-made missile systems which he said were being channelled to the separatists: "Tanks, howitzers, Grad systems, Smerch, Buk".

"Radio-electronic surveillance stations are not on sale in the Donetsk market - they are only to be had from the Russian defence ministry and Russian military intelligence," he said.

Ukrainian officials have insisted Moscow sticks to the 12-point peace plan agreed in Minsk in September.

They say that plan was not violated by its airport counter-offensive, launched after troops had appeared to be pinned down inside the complex.

Ukrainian military spokesman Andriy Lysenko said the situation was still very tense around the airport, and separatists continued attacks on government forces there and elsewhere in the east.

Since plans for another round of peace talks last week were abandoned, fighting has flared up again in Ukraine.

Ukraine's Crimean peninsula was annexed by Russia in March last year, prompting a crisis with the West, which has imposed sanctions.

The World Health Organisation says more than 4,800 people have been killed in the conflict.

In Brussels, European Union foreign ministers said now was not the time to ease the economic sanctions against Russia, despite conciliatory proposals from the EU's foreign policy chief Federica Mogherini.

Ms Mogherini had suggested that member states could start talking to Russia again on a range of issues if Moscow implemented peace agreements.

However, other countries said this would send the wrong message to Mr Putin that the EU's resolve was cracking.