Jacob Resneck and Marc-André P. Richer-Henry

Special for USA TODAY

SLOVYANSK, Ukraine — The mayor of a major city that has been a flashpoint of pro-Russian militancy was shot in the back Monday as hundreds of men attacked peaceful protesters who support a united Ukraine.

Tensions escalated in East Ukraine with pro-Russia separatists seizing another government building and Kharkiv mayor Hennady Kernes shot Monday morning in the back.

Kernes underwent surgery and was in "grave" condition according to officials in this city of more than 1 million people. It was not known who was behind the shooting.

Russia's defense chief meanwhile assured U.S. Defense Secretary Chuck Hagel in a telephone call that Russia would not invade Ukraine, the Pentagon said.

Hagel spoke Monday with Russia's defense minister, Sergei Shoygu, who reassured Hagel that Russian forces would not invade Ukraine, Navy Rear Adm. John Kirby said in a statement. Kirby said Hagel had requested clarification of Russia's intentions in eastern Ukraine. /

Ukraine's government once accused Kernes of organizing activists to go to Kiev to harass demonstrators there who succeeded in ousting pro-Russian president Viktor Yanukovych. But Kernes had recently had a change of heart about the situation in Ukraine, saying he does not support the pro-Russia separatists or the annexation of Ukrainian territory.

In the eastern city of Donetsk, a peaceful pro-Ukraine rally of about 1,000 people chanting "Donetsk is Ukraine!" was broken up by masked, bat-wielding men in military fatigues in the worst street violence here since separatists began occupying administrative buildings earlier this month.

Donetsk's municipal authorities said 14 people had been seriously injured.

"The police are supposed to protect us," said Espen, 19, a university student who didn't want his last name published out of fear of reprisal.

Elsewhere, pro-Russia militants wearing masks seized a city hall building and police station in the city of Kostyantynivka, 100 miles from the Russian border. Western nations accuse Russia of supporting separatist gunmen and imposed fresh sanctions against Russian individuals and companies Monday.

On Sunday the commander of the pro-Russia militants in the eastern city of Slovyansk identified himself as Igor Strelkov, who Ukraine says is a Russian intelligence officer in Ukraine on orders from Moscow to create an uprising against the elected government.

"The unit that I came to Slovyansk with was put together in Crimea. I'm not going to hide that," Strelkov told Moscow-based Komsomolskaya Pravda in a video interview. "It was formed by volunteers. I would say half or two-thirds of them are citizens of Ukraine."

Meanwhile, seven European military observers remain held hostage by armed separatists in eastern Ukraine following the release of one of the men for health reasons. The head of the German-led delegation said he believed the group, captured Friday, is being used as a bargaining chip.

The men, part of the Organization for Security and Cooperation in Europe, were paraded in front of the reporters Sunday where, under armed guard, they said they had not been mistreated. Western leaders were outraged.

"The public parading of the OSCE observers and Ukrainian security forces as prisoners is revolting and blatantly hurts the dignity of the victims," said German Foreign Minister Frank-Walter Steinmeier.

The remaining captives, from Germany, the Czech Republic, Denmark and Poland remain in the hands of pro-Russia separatists in Slovyansk where militia leaders have indicated they intend to swap them for members of their own movement currently imprisoned by Ukraine.

To date, Ukraine's interim government has refused to negotiate with the separatists who they accuse of being puppets of Russia trying to destabilize the country.

The separatists in Slovyansk have taken hostages, including journalists and pro-Ukraine activists, as they strengthen their control in the east of the country in defiance of the interim government in Kiev and its Western supporters.

So far Kiev's efforts to regain control have been ineffective. The self-declared mayor of Slovyansk, Vyachislav Ponomaryov, vowed that his armed men — who he insists are a mix of volunteers and locals from neighboring countries including Russia — would continue to resist efforts by the Kiev government to retake the town held for three weeks.

"We are on our own land and we are going to protect our land as our ancestors did," Ponomaryov said.

His movement has occupied about 10 administrative buildings in the region and vows to hold a May 11 referendum over secession, which Russia has been insisting he and others be allowed to do. While not explicitly inviting Russian intervention from the estimated 40,000 troops currently massed on the border, Ponomaryov said that day could come.

"If the Russian government sees the situation getting out of control, I wouldn't be against having Russian troops here."

It is statements like this, coupled with Russian President Vladimir Putin's implicit threat that Russia could intervene to protect its interests in the region, that have led officials in Kiev to accuse Moscow of trying to start a World War III.

But in Slovyansk there are those who say while having armed men occupying buildings is bad the prospect of Kiev sending troops is worse.

"I don't like how this separatist movement seizes buildings — they could work in other ways," Heinrich Jakobi, 75, a retired welder who has lived in Slovyansk for decades, told USA TODAY. "But it'd be even worse for the military to send in their troops."

His wife, a retired accountant also in her 70s, said she thought the Russian threat had been exaggerated.

"There are no Russians here," she said. "People say that all the time but they're not and it's just increasing tensions."

Her husband repeated that there are no military solutions to the crisis and that the so-called terrorists are actually local citizens.

"I don't like these separatists but to send in the army against the people would be even worse — that'd be awful," he said. "It could lead to civil war."

Earlier Ukrainian security officials sought to assure people they would not assault the town center which has been heavily fortified by militants armed with machine guns, armored personnel carriers and rocket-propelled grenades.

"We do not want any casualties. We will not storm the city," Vasul Krutov, deputy head of the Security Service of Ukraine, told reporters in Kiev, Interfax-Ukraine reported. "We realize that there may be a large number of injured people in this case."

Contributing: Associated Press