A dangerous face-off in eastern Ukraine between government forces and shadowy pro-Russian separatists appears more and more likely to determine whether the country can hold together - even without a Russian military invasion.

Authorities in Kiev insist that Russia is looking for a pretext to invade, and they are striving not to provide one.

The Russian government dismisses any suggestion that it intends to intervene and says Ukrainians should stop blaming Moscow for their problems.

But Russian troops remain near the border conducting exercises, and Secretary of State John F. Kerry echoed the Ukrainian allegation that Russians are fomenting trouble.

"It is clear that Russian special forces and agents have been the catalyst behind the chaos of the last 24 hours," Kerry said in testimony before a Senate committee. He said the unrest "could potentially be a contrived pretext for military intervention just as we saw in Crimea" and warned of harsh new sanctions if Russia invades.

Right-wing Russian nationalist groups have been seeking volunteers through social media to go to Ukraine, but it is not clear whether those appeals have borne fruit.

Pro-Russia protesters in three eastern Ukrainian cities simultaneously attacked government buildings Sunday night. In Kharkiv, they were ousted by police early Tuesday. But in Donetsk and in Luhansk, the separatists have held tight. They have declared a "people's republic" but have made it clear that they favor Crimean-style annexation by Russia.

In Donetsk on Tuesday afternoon, more than a dozen passengers on a flight from Moscow were pulled aside at passport control as Ukraine attempts to bar those it suspects of causing trouble.

As evening fell and people got off from work, the crowd outside the occupied regional administration building in the city swelled to a few thousand. Barricades of tires, automobile bumpers, barbed wire and sandbags rose along the perimeter of the 11-story late-Soviet slab of a building.

Police presence appeared extremely light, and those officers present, in regular uniforms, made no effort to restrict access to the site. Women on the plaza in front of the building merrily chanted "Together we're here to the end" and "Donetsk is a Russian city."

A group broke into song, belting out a World War II favorite about Katyusha rockets.

But a block away, traffic moved as usual. This is not a city in the grip of secession fever - or anti-secession fever. On a warm spring afternoon, the playgrounds were full of children, couples strolled, shoppers shopped. Politics stops about 100 yards from the center of the action.

And, yet, the fate of the country hangs in the balance.

Donetsk, about 50 miles west of the Russian border, has strong ties, economic and otherwise, to Russia, and distaste for the new government in Kiev runs strong here. But support for secession appears to be thin among the city's 1 million residents.

On Monday evening, one of Ukraine's richest men, the coal baron Rinat Akhmetov, met with protesters and, although expressing sympathy and support, urged them to talk with the Kiev government. He strongly denies allegations surfacing in the Ukrainian news media that he has helped finance the pro-Russia demonstrations.

Yet it's difficult to know, as the two sides become more polarized, how the crisis might play out.

Protesters were cleared overnight from the regional administration building in Kharkiv, but others held out in two more eastern cities yesterday in what Kiev says is a Russian-led plan to dismember the country.

Shots were fired, a grenade thrown and 70 people detained as officers ended the occupation in the city of Kharkiv during an 18-minute "anti-terrorism" action, Ukrainian Interior Minister Arsen Avakov said.

The West has been warning Russia, which annexed Ukraine's Crimean Peninsula last month, against any incursion into eastern Ukraine. "If Russia were to intervene further in Ukraine, it would be a historic mistake," NATO Secretary General Anders Fogh Rasmussen told reporters Tuesday in Paris. "It would have grave consequences for our relationship with Russia and would further isolate Russia internationally."

Russia blames U.S.

In Moscow, Russian Foreign Minister Sergei Lavrov said Tuesday that the United States, not Russia, was responsible for sowing discord in Ukraine. "Our American partners are trying to assess the situation," Lavrov told reporters, "applying their habits to others."

The Russian Foreign Ministry charged that ultranationalists from Ukraine's Right Sector movement and American mercenaries were among the police force Kiev sent to eastern Ukraine to quell the violence.

"We are particularly concerned that the operation involves some 150 American mercenaries from a private company Greystone Ltd., dressed in the uniform of the [Ukrainian] special task police unit Sokol," the ministry said in a statement posted on its Web site Tuesday morning. It called for an immediate halt to "all military preparations which could lead to a civil war."

Ukrainian officials denied that any mercenaries or irregular forces are at work in eastern Ukraine.

"There is no Right Sector, let alone U.S. security forces, in Kharkiv, Donetsk or Luhansk," Serhiy Pashynsky, chief of the presidential administration in Kiev, said Tuesday. The Ukrainian Foreign Ministry issued a similar denial.

Earlier reports in Russian news media identified Greystone as a subsidiary of the private security firm once known as Blackwater and later renamed Academi.

Two weeks ago, Russia's Itar-Tass news agency quoted Ukrainian government security sources as saying that they intended to hire private military personnel from Greystone "to suppress" the eastern, Russian-speaking regions of Ukraine. In early March, Russian state television reported that several hundred armed Greystone employees had flown into the Kiev airport.

In Washington, a senior Pentagon official told a House committee Tuesday that the United States is extending the stay of the destroyer USS Truxtun in the Black Sea and will send another ship there in a week. The Truxtun was dispatched last month to conduct training with the Romanian and Bulgarian navies, a mission scheduled before the Ukraine crisis erupted.