The new governor of the eastern Ukrainian region of Donetsk on Friday said Russians were behind violent clashes between rival demonstrators in which one man was killed, and accused Moscow of distorting the truth in its account of what happened.
Russia warned it could move in to protect compatriots – a similar justification as used in last week's military takeover of Crimea. And Ukraine's acting president raised the alarm over a Russian troop build-up on the eastern border that has fuelled fears in Kiev of a broader invasion by its former Soviet ruler.
Serhiy Taruta, a steel tycoon and one of several oligarchs appointed to take control of possibly restive, Russian-speaking regions after last month's overthrow of Ukraine's Moscow-backed president, scoffed at the Russian Foreign Ministry's implication that Russians had been victims of Thursday night's violence.
"Sadly, we note that there were, according to police, a lot of people concentrated there who were not from Ukraine," he told reporters in an oblique reference to Russia and to Ukrainian accusations that Moscow has sent militants across the border to stir up violence in eastern cities since the fall of its ally.
Late on Friday, Ukrainian media said two people were killed in the northeastern city of Kharkiv in clashes involving Russian nationalists, identified by ribbons commemorating St George, and an unknown group. Five people were injured.
The reports, quoting the local prosecutor, were accompanied by video footage of gunfire and explosions. They said the nationalists attacked a group of offices, including one belonging to a Ukrainian cultural association.
Earlier fierce clashes in Kharkiv had prompted city authorities to issue appeals for calm during this weekend.
Russia's Foreign Ministry, referring to the clashes in Donetsk, heart of the Donbass coalfield, said the unrest showed Ukraine's authorities had lost control. It added: "Russia is aware of its responsibility for the lives of compatriots... and reserves the right to take people under its protection.
However, activists from groups which favour Ukraine's integration with western Europe and which demonstrated on Thursday against Russia's occupation of Crimea, said the dead man and most of the injured in Donetsk were from their side.
The right-wing party Svoboda said Dmitriy Chernyavskiy, 22, who was stabbed to death, was one of its organisers in Donetsk.
Taruta said: "The statement by the foreign ministry is not objective and distorts the real situation."
Journalists saw pro-Russian demonstrators throw eggs, smoke bombs and other missiles and break through a police cordon to beat rival demonstrators with batons. Organisers of the pro-Russia protest said their supporters were also attacked.
The interior minister said four "ringleaders of the mass disorder" had been arrested, adding: "We will not go easy on bandits with knives."
The violence was the deadliest in Ukraine since Moscow-backed president Viktor Yanukovich fled the country last month after police shot dead dozens of demonstrators angered by his rejection in November of an alliance with the European Union.
That was a setback for Russian President Vladimir Putin, who had hoped to bring Ukraine into a customs union involving other big ex-Soviet states. Moscow denounced a "coup" in Kiev and has taken control of Crimea, ostensibly to protect its ethnic Russian majority from "fascist" rulers in Kiev. Crimeans will vote on Sunday in a referendum on uniting with Russia.
Despite intense international diplomatic pressure and threats of economic sanctions if Russia goes ahead with the vote on annexation, Putin has shown no sign of stepping back in what is the gravest security crisis in Europe since the Cold War.
That has fuelled concern in Ukraine that Russian troops, engaged in exercises on its mainland borders, could move in, possibly using street violence in eastern cities to justify it.
There have been scuffles in other cities in the east, including shortlived attempts by pro-Russian activists to raise the Russian tricolour on public buildings.
Acting president Oleksander Turchinov, who reviewed large Ukrainian military exercises on Friday, has said the massing of Russian troops showed they were "ready to invade ... at any moment".
Russian Foreign Minister Sergei Lavrov said on Friday that Moscow had no plans to invade southeastern Ukraine.