Foreign Minister Sergei Lavrov's warning about Russian intervention was the most explicit declaration yet of his country's intentions toward eastern Ukraine. It came as Vice President Joe Biden arrived in Kiev for a two-day visit to meet with political leaders, civil society groups and U.S. diplomats.
Biden brought a commitment of U.S. economic assistance. He also warned Russia that future intervention in Ukraine's volatile east will bring new costs.
But the Russian threats held out the possibility that Moscow will be on the move - and soon.
"Those who are deliberately pursuing a civil war, in a possible attempt to start a big, serious bloody conflict, are pursuing a criminal policy," Lavrov said. "And we will not only condemn this policy, but will also stop it."
A tentative agreement reached Thursday in Geneva by Ukraine, Russia, the European Union and the United States has gained little traction. The armed pro-Russian forces in eastern Ukraine have not abandoned the buildings they have occupied, defying the pact.
Geoffrey Pyatt, the U.S. ambassador to Ukraine, said Monday evening that Washington wants to see movement under the accord in "days, not weeks."
Lavrov, in contrast, lashed out at the new Ukrainian government for "flagrantly" refusing to dismantle the protest camp at Kiev's Independence Square, known as the Maidan, nearly 350 miles west of the disputed regions. It was the epicenter of the months-long protest against now-ousted President Viktor Yanukovych. Lavrov demanded that it be dispersed at once as a prerequisite for further de-escalation.
On Sunday, three people were killed in a shootout in the eastern city of Slovyansk. Lavrov said the residents of the Donetsk region craved Russian protection.
"There has been a surge in appeals to Russia to save them from this outrage," he said at a news conference here, invoking a time-honored Russian rationale for military intervention in another country. "We are being put into an extremely complex position."
As Russia shook its fist, citizens in eastern Ukraine who want to remain unified under a central government in Kiev began to publicly oppose the pro-Russian agitators.
In the port of Mariupol and the industrial city of Khartsyzk, residents staged some of the first "pro-unity" rallies in the area.
Protected by a low-key police presence, businessman Vyacheslav Redko stood before a crowd in front of the Soviet-era Palace of Culture in Khartsyzk and demanded that authorities take down the Russian flag now flying above City Hall and remove the barricade of tires at the entrance.
"It is a myth that everyone here wants to join our big brother Russia," Redko said afterward. "But our side has not been heard, because people are scared or intimidated or they assume they cannot win."
Redko said he was surprised to see a few hundred of his neighbors at the rally. He would have been happy with a few dozen. "For Khartsyzk? This is a big deal," he said.
A dozen people mounted the steps of the palace. One elderly pensioner sang a popular Ukrainian folk song. A retired teacher of Russian literature complained that his former students, now pro-Russian activists, had roughed him up for supporting Ukrainian unity.
"We're afraid of a war with Putin. We know his troops are on the border. But I think the more people see that they are not alone, the bigger these rallies will become," said Lyudmila Pogromskaya, an English-language instructor, referring to the Russian president.
Vladimir Ponomaryov, "the people's mayor" of Slovyansk, told Russian reporters that self-defense militia members in the city have detained 20 people suspected of spying for Kiev. One of those was Ukrainian journalist and activist Irma Krat, who was shown on Russian TV blindfolded and being escorted by local militia members.
He also said that two bodies have been pulled from the river that runs through the city. The victims, members of the pro-Russian Donestk People's Republic, had been stabbed to death, he said. It was not possible to confirm the claim.
Biden, whose main meetings in Kiev are scheduled to take place Tuesday, is the most senior Obama administration official to visit Ukraine since its crisis with Russia began two months ago, leading to Russia's annexation of Ukraine's autonomous Crimea region in March.
The challenge presented by pro-Russian groups in eastern Ukraine has become "the most acute" facing Ukraine's leadership, said a senior U.S. official traveling with Biden.
"There are currently ongoing threats to Ukraine's sovereignty and territorial integrity, and the most effective response to that is for all of Ukraine to pull together," the official said, speaking on the condition of anonymity to discuss the trip.
Pyatt said Ukrainians "are now wondering what comes next and are looking for reassurance that the United States is going to stand by Ukraine."
Biden is expected to announce a new U.S. technical support package aimed at boosting Ukraine's economy, energy sector and political reform efforts in the run-up to presidential and mayoral elections next month.
In the Russian city of Rostov-on-Don, just over the border from Ukraine, Yanukovych called Monday for a pullback of Ukrainian forces from the east and the holding of regional referendums on federalization of the country before presidential and parliamentary elections are held.
Otherwise, Ukraine risks civil war, he said in a statement released to news agencies. The referendums should be held under an international system of control and observation, he said.
"This is the only way to preserve the integrity of Ukraine," his statement said. "If this is not done, our country will be threatened with fragmentation, which would hardly be a peaceful process. A civil war would be more likely. Come back to your senses! You are setting brother against brother, while somebody else would take advantage of this disaster."