Ukrainian armed forces yesterday launched a "special operation" against militiamen in the country's Russian speaking east, authorities said, recapturing a military airfield from pro-Moscow separatists.
Gunfire could be heard from the airfield at the town of Kramatorsk after a fighter jet swooped low over the area. Ukrainian troops were seen disembarking from helicopters.
Russia said it was deeply concerned over reports of the military intervention in eastern Ukraine.
Watch: Tension in Kramatorsk, eastern Ukraine
"The reports we are getting cause deep concern. To all appearances, events are beginning to develop under the worst case scenario," Konstantin Dolgov, the Russian foreign ministry's human rights representative, was quoted by state news agency RIA as saying.
The operations in Kramatorsk and elsewhere appeared to mark an escalation of the deepest East-West crisis since the Cold War. The standoff has raised fears in the West and in Kiev that Russia might intervene militarily on behalf of Russian speakers in eastern Ukraine.
Ukraine's acting President Oleksander Turchinov had earlier announced that a military operation was under way to flush pro-Russian separatists out of the government buildings and facilities they have seized in about 10 towns and cities in the east over the last few days.
Turchinov issued a statement saying Ukraine had retaken the airfield in Kramatorsk from pro-Russian militants, while the state security service said an "anti-terrorist" operation was in progress against separatists in the nearby town of Slaviansk.
In the US, White House spokesman Jay Carney said that Ukraine faced an "untenable" situation and described its military operation against pro-Russia militants as "measured."
Russian foreign minister Sergey Lavrov warned Kiev against using force , saying the "criminal" act would undermine talks planned in Geneva.
The four-way meeting set for tomorrow involving top diplomats from Russia, Ukraine, the United States and the European Union is the latest step in a flurry of diplomacy.
In unusually strong language at a joint news conference in Beijing with his Chinese counterpart Wang Yi, Lavrov denounced Kiev for "spreading lies" about Russia's position and actions in Ukraine.
Moscow "in principle" supports the idea of four-way talks, although they must be "genuine and not merely for show," Lavrov said.
"So, if the Ukrainian foreign ministry says Russia is afraid to hold these talks in Geneva, don't believe it," he said. "It's a lie."
Lavrov also denied international claims that Russia is dispatching pro-Kremlin forces to Ukraine's southeast.
"This is a total lie that supposes that those residents there are completely incapable of protesting of their own will," he said.
Lavrov's visit, which included a meeting later yesterday with President Xi Jinping, comes amid preparations for Russian President Vladimir Putin's trip to China next month, as the two powers forge increasingly close ties.
Beijing has refrained from publicly criticising Russia on the issue. And when the UN General Assembly last month adopted a Ukraine-backed resolution condemning Crimea's referendum and refusing to recognise Russia's annexation of the peninsula, China abstained from voting.
Lavrov told reporters at the news conference with Wang that Russia "appreciates China's objective, balanced and responsible stance" on the Ukraine issue.
Echoing comments he made on Monday at a meeting with German Foreign Minister Frank-Walter Steinmeier, Wang said China welcomes this week's four-way talks in Geneva.
"We're optimistic this will advance the cause of peace that we advocate," Wang said.
Additional reporting by Agence France-Presse