Ukraine’s interim government announced today that it would use force to wrest control of government buildings in its eastern provinces away from pro-Russian separatists. At the same time, Russia warned that any such use of force would amount to a “civil war” within Ukraine, with an unspoken threat of intervention on behalf of ethnic Russians hanging in the air:

The Ukrainian government announced the start of a staged counteroffensive Tuesday to reclaim control of the eastern part of the country, as Russian Prime Minister Dmitry Medvedev warned that Ukraine was descending into civil war.

Facing mounting pressure to act following the takeover of official buildings by pro-Russian separatists in at least nine cities in the restive east, acting Ukrainian President Oleksandr Turchynov said that a counterterrorism operation had begun in the northern Donetsk region early Tuesday.

Eyewitnesses and officials reported a buildup of Ukrainian forces not far from Izyum, a city located near the border of Kharkiv and Donetsk provinces in the east. The area sits a few dozen miles north of Slavyansk, which was seized by well-armed pro-Russian activists Saturday and was the site of a failed operation by Ukraine to reclaim the city on Sunday.

An Izyum official involved in the mobilization and who asked not to be named said the city was being used as a fueling and feeding station for Ukrainian troops, who had begun arriving over the weekend and were taking up positions outside the city’s limits.

So far, though, not much is happening:

Russia declared Ukraine on the brink of civil war on Tuesday as Kiev said an “anti-terrorist operation” against pro-Moscow separatists was underway, though the crackdown got off to a slow start, if at all.

Twenty-four hours after an Ukrainian ultimatum expired for the separatists to lay down their arms, witnesses reported no signs yet that Kiev forces were ready to storm state buildings in the Russian-speaking east that the rebels have occupied.

Police said separatists had voluntarily surrendered the police headquarters in the city of Kramatorsk.

Interim president Oleksander Turchinov insisted the operation had started in the eastern Donetsk region, although it would happen in stages and “in a considered way”.

Russian foreign minister Sergei Lavrov said a use of force threatened the planned summit meeting between Russia, Ukraine, and the West that intended to find a peaceful settlement of the crisis:

The four-way meeting set for Thursday involving top diplomats from Russia, Ukraine, the United States and the European Union is the latest step in a flurry of diplomacy aimed at easing the worst European security crisis in decades.

“One cannot issue invitations to talks while at the same time issuing criminal orders for the use of armed force against the people there,” Lavrov said during a visit to Beijing.

“You can’t send in tanks and at the same time hold talks, and the use of force would sabotage the opportunity offered by the four-party negotiations in Geneva,” he said.

The government in Kyiv had earlier scoffed at Russia’s participation in the summit, which drew a rather undiplomatic response from Lavrov:

In unusually strong language at a joint news conference with his Chinese counterpart Wang Yi at Beijing’s Diaoyutai State Guesthouse, 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.”

Meanwhile, Barack Obama took a phone call from Vladimir Putin yesterday, which was described as “frank and direct” — and that usually means “unhappy.” Obama is still considering a new list of sanctions targets, including another member of Putin’s “crony network,” but not economy-wide sanctions — yet:

Judging by public accounts, the latest telephone call between Mr. Obama and Mr. Putin did little to bridge the divide. In the call, requested by Moscow, Mr. Obama accused Mr. Putin of trying to subvert Ukraine by fomenting takeovers of security buildings. “The president expressed grave concern about Russian government support for the actions of armed, pro-Russian separatists who threaten to undermine and destabilize the government of Ukraine,” the White House said in a statement.

Mr. Putin denied interfering in Ukraine. “Such speculations are based on inaccurate information,” the Kremlin said in a statement. Instead, he placed blame for the turmoil on Ukraine’s new pro-Western government and its “unwillingness and inability to take into account the interests of the Russian and Russian-speaking population.” …

After a weekend of meetings, the White House was working on a list of new targets who would be barred from traveling to the United States or whose assets here would be frozen, officials said. Among them is Igor Sechin, a longtime Putin ally and the president of Rosneft, the largest state-owned Russian oil company, which has a major joint venture with ExxonMobil. European ministers agreed on Monday on a list of their own.

The American sanctions would also be imposed on at least one Russian institution that is deemed part of the so-called crony network supporting Mr. Putin, officials said. But they added that Mr. Obama does not plan to place more crippling measures on whole sectors of the Russian economy unless the Kremlin escalates its actions. The White House wants to hold those sanctions in reserve should Moscow invade Ukraine or seek to annex its eastern regions.

Perhaps everyone right now is jockeying for position for Thursday’s summit. That may be Ukraine’s best opportunity to get the separatists out of their buildings without bloodshed, but it’s a long shot in any case.