The mayor of Ukraine's second-largest city has been shot in the back and pro-Russian insurgents have seized more government buildings in the country as unrest continued.
The US has also hit Russia with more sanctions for allegedly fermenting the unrest in eastern Ukraine.
One presidential candidate said the mayor was deliberately shot in an effort to destabilise the entire city of Kharkiv.
Armed insurgents tacitly backed by Moscow are seeking more autonomy in the region and possibly even independence or annexation by Russia.
Ukraine's acting government and the West have accused Russia of orchestrating the unrest, which they fear Moscow could use as a pretext for an invasion.
Ratcheting up the pressure on Russian president Vladimir Putin, US president Barack Obama's government levied new sanctions on seven Russian officials and 17 companies with links to Putin's inner circle.
The US has also revoked licenses for some high-tech items that could be used by the Russian military.
In Brussels, the European Union has moved to add 15 more officials to its Russian sanctions list to protest against Moscow's meddling in Ukraine.
Hennady Kernes, the mayor of Kharkiv, was shot in the back this morning while cycling on the outskirts of the city and underwent surgery, city hall said.
He was reported by the hospital to be in "grave, but stable" condition.
Officials have not commented on who could be behind the attack but Mr Kernes was a man who could have angered both sides.
His friend and former Kharkiv governor, Mykhailo Dobkin, said the attackers were aiming at Mr Kernes' heart and wanted to kill him in an effort to destabilise the city of 1.5 million.
Mr Dobkin is among several candidates running in Ukraine's May 25 presidential election. "If you want to know my opinion, they were shooting not at Kernes, but at Kharkiv," he said.
Mr Kernes was a staunch opponent of the pro-West Maidan movement that toppled president Viktor Yanukovych in February and was widely viewed as the organiser who sent activists to Kiev from eastern Ukraine to harass those demonstrators.
But he has softened his stance toward the new Kiev government. At a meeting of eastern Ukrainian leaders and acting Prime Minister Arseniy Yatsenyuk this month, Mr Kernes insisted he does not support the pro-Russia insurgents and backed a united Ukraine.
Kharkiv is in eastern Ukraine, where pro-Russia gunmen have seized government buildings and police stations, set up roadblocks and staged protests.
But unlike the neighbouring Donetsk region, Kharkiv has been largely unaffected by the insurgency and Mr Kernes has been credited for this. Its administration building was briefly seized earlier this month but promptly cleared of pro-Russia protesters.
The Russian Foreign Ministry said the attack on him, along with other events, "indicates that it isn't possible to speak of any 'peaceful' pre-election campaign in Ukraine".
Pro-Russia militants wearing masks gained another foothold in east Ukraine today, seizing a city hall building and police station in the city of Kostyantynivka, 100 miles from the Russian border.
The city is 22 miles south of Slovyansk, a major city that has been in the hands of insurgents for more than three weeks.
After the seizure, about 15 armed men guarded the city hall building. Some posed for pictures with residents while others distributed St George's ribbons, the symbol of the pro-Russia movement.
Moscow has repeatedly pushed for a referendum on federal autonomy in Ukraine, but Kiev and its Western allies have refused, accusing Russia of fomenting separatist sentiment in an attempt to foil the May presidential vote.
However, Justice Minister Petro Petrenko said the parliament in Kiev would debate the idea of a referendum tomorrow, Interfax news agency reported.
The increasingly ruthless pro-Russia insurgency, meanwhile, is turning to an ominous new tactic: kidnapping.
About 40 people are being held hostage in makeshift jails in Slovyansk - including journalists, pro-Ukraine activists and seven military observers from the Organisation of Security and Cooperation in Europe, Ukraine's Security Service said.
The German government called for the immediate release of the European military observers, who were detained on Friday amid allegations they were spying for Nato.
Pro-Russia militants in camouflage and black balaclavas paraded some of the captive military observers before the media yesterday. They also showed three Ukrainian security guards bloodied, blindfolded and with their arms bound.
On the other side, Russia's foreign ministry said Ukraine's effort to detain pro-Russia activists had become a "witch hunt" that involved the "mass persecution of dissenters."
The foreign ministry also said it had information that Ukraine was building mass temporary detention centres for these prisoners.
"Those structures being constructed very much remind one of fascist concentration camps," the Russian statement said.