A close political ally of Vitali Klitschko, Kiev’s new mayor and a former world champion boxer, was killed yesterday fighting Russian-backed separatists in east Ukraine – the first prominent leader of the Maidan revolution that toppled President Viktor Yanukovych to die battling insurgents.

Killed by a sniper 30 miles south of the city of Donetsk, Mykola Berezovy, 37, was married to a Kiev revolutionary heroine, Tetyana Chornovil, a journalist-turned-activist who did much to expose the corruption of the Yanukovych family and was beaten nearly to death herself by riot police during the uprising.

Berezovy's death brings to 568 the number of Ukrainian soldiers and volunteer militiamen who have been killed fighting in the country’s eastern-most provinces of Donetsk and Luhansk, according to Ukraine’s security services spokesman Col. Andriy Lysenko, who says 2,100 others have been wounded in battles and skirmishes that have raged now full-tilt since the election in May of Petro Poroshenko as the country’s president.

Poroshenko vowed to restore peace in the industrial Donbas region quickly, but that is proving a hard task.

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UN officials say at least 1,100 civilians have been killed since the Kiev government launched its “anti-terrorist” operations aimed at reclaiming dozens of cities and towns seized by separatists. Lysenko also said that Russia has routinely breached Ukrainian airspace with over-flights by drones and other aircraft, some flown from Crimea, the Black Sea peninsula seized by Moscow earlier this year. Ukraine accuses Russia of sending operatives to join the separatist ranks, despite Kremlin claims that it is not fomenting trouble.

NATO officials claim that Russian President Vladimir Putin has massed troops once again along the border with Ukraine, estimating that 20,000 Russian soldiers are now deployed.

Berezovy was platoon commander in a volunteer force known as the Azov battalion.

“Our platoon was following an armored personnel carrier and was targeted by snipers,” according to another volunteer, Mykola Lyahovych. Writing on his Facebook page, Lyahovych maintained the snipers were “Russian recruits — professionals.” Berezovy, who had the nom de guerre Falcon, was trying to rescue another pro-Kiev volunteer when he was shot. He bled out before medical help could arrive, sources said.

His death will likely fan the anger of volunteer battalions, who are keen to launch a final assault on the main rebel stronghold in central Donetsk. But Poroshenko's administration seems cautious about how to approach the retaking of the city, the capital of the Donbas region.

In recent days, the military says it has reclaimed three-quarters of the territory separatists seized, expelling insurgents from key towns of Kramatorsk and Slovyansk, once the headquarters of Igor Strelkov, a Muscovite whose real name is Igor Girkin and who Ukrainian security officials say is a Russian GRU military intelligence officer.

Donetsk is now encircled, setting up the final stage for a possible bloody urban showdown. Ukrainian military officials say they are ready to unleash a final assault on Donetsk and Luhansk, cities that were home to 1.5 million people before the insurgency.

“We’re asking civilians to leave the cities where possible,” spokesman Lysenko told reporters here in Kiev this morning. “The operation to encircle these cities is almost over. The active operation continues day and night. We’re pushing ahead and aren’t stopping.”

But similar warnings of an imminent final assault on the few thousand separatist fighters holed up in both cities have been issued before, but the Ukrainian military has held off. Sources on the Ukrainian National Defense Council tell Fox News there is a lively debate with the government about whether a large-scale assault should be mounted or whether a more piecemeal strategy should be pursued.

Those who favor caution are concerned about the reaction of Russia and whether Putin uses a major assault as a pretext to intervene. Moscow appears to be laying the rhetorical framework for such an intervention. Kremlin officials talked over the weekend of mounting a “humanitarian mission,” and a military column was spotted closing in on the border, say Ukrainian spokesmen.

Valeriy Chaly, the deputy chief of Poroshenko’s administration, claimed after a frenzy of diplomatic activity involving the Americans and Europeans, the Russian military convoy halted at the border. That was accompanied by a half-hearted offer by separatist leaders of a cease-fire that was withdrawn hours later. Moscow denies that it intended to cross the border, but NATO officials said that the Kremlin might use a humanitarian mission as an excuse to invade eastern Ukraine.

“The tragic humanitarian situation that’s unfolding in the region can’t wait,” Dmitry Peskov, Putin’s spokesman, said today (Monday) in the Black Sea resort of Sochi.

European Union chief Jose Manuel Barroso warned Putin not to carry out unilateral military action in eastern Ukraine under an aid pretext. The two leaders spoke over the phone as the Kremlin announced it was working with the Red Cross on sending a humanitarian aid convoy to the region.

Whether Moscow’s threats of intervention are serious or just part of a game of bluff remains unclear. But the humanitarian situation is worsening in the besieged cities of Donetsk and Luhansk -- where armed clashes and shelling continued apace today. Nearly 250,000 people in Luhansk have been without running water and electricity for two weeks, and landline and cellphone communications are now failing. Luhansk residents say stores are mainly closed, banks are not functioning and pensions and salaries are unpaid.

The picture is not dissimilar in Donetsk, residents say, with food stores running low. However, much of the public transportation system is still working, although buses have to be re-routed because of artillery damage. Fuel stocks are running low, too. And the center of Donetsk is mostly deserted at night.

An artillery shell on Monday hit a “strict regime” high-security prison in Donetsk, killing an inmate and allowing 106 other convicts, rapists and murderers included, to break out. Ukrainian officials insist government forces did not fire the shell.

Criticism is mounting of Poroshenko from easterners at the absence of any systematic state support for the thousands who have fled -- they have had to rely on charity and the aid of friends and family.

“There has to be a humanitarian strategy to help the refugees,” says Mykola Volynko, head of the Donbas Independent Miners trade union. “People can’t get their pensions and can’t even register at hospitals in the rest of the country because they are told they have to have their medical notes from Donetsk and they are impossible to get.”

In Kiev, though, there is growing resentment toward refugees from the east, with locals here saying that while their sons and husbands are dying, easterners are not pulling their weight in the battle against the separatists.