Russian President Vladimir Putin has warned that Ukraine is heading into an "abyss" by confronting pro-Russian separatists in the east of the country.
He said claims that Russian agents were acting in the region were "rubbish".
He was speaking live on Russian TV after a clash in Mariupol, eastern Ukraine, in which three separatists were reportedly killed.
Talks on the crisis are now under way in Geneva between Ukraine, Russia, the European Union and the US.
Russia and Ukraine have been on a collision course since pro-Moscow Ukrainian President Viktor Yanukovych was toppled in February by supporters of more engagement with the West, in particular the EU.
Tensions escalated last month when Russia annexed Crimea - causing international outrage - following protests against the new government in Kiev by its majority Russian-speaking population.
Unrest then spread to the Donetsk region and other parts of eastern Ukraine bordering Russia.
Following Wednesday's events in Mariupol and confrontations in other parts of eastern Ukraine, the country's State Border Service announced on Thursday that it was "significantly" restricting entry into the country by adult men from Russia because of the risk of "acts of terror".
Mr Putin made his latest comments in a televised question-and-answer session with the Russian public, a talk-show-style programme involving satellite links with audiences across Russia that has become an annual event lasting several hours.
"Instead of realising that there is something wrong with the Ukrainian government and attempting dialogue, they made more threats of force... This is another very grave crime by Kiev's current leaders," he said.
"I hope that they are able to realise what a pit, what an abyss the current authorities are in and dragging the country into."
Mr Putin pointed out that he had secured authorisation in March from Russia's upper house of parliament to use force in Ukraine, but he stressed that he preferred negotiations.
"The Federation Council granted the president the right to use military force in Ukraine. I really hope that I do not have to exercise this right and that we are able to solve all today's pressing issues via political and diplomatic means," he said.
"We must do everything to help these people [in eastern Ukraine] defend their rights and independently determine their own destiny. This is what we're going to push for."
Responding to Mr Putin's comments, Ukraine's prime minister accused the Russian president of attempting to sabotage the country's forthcoming presidential election, and said Moscow was responsible for deaths in recent clashes in the east.
"Russia is playing only one game - further aggravation, further provocation - because the task... is to wreck the presidential election on May 25," Arseny Yatseniuk told journalists in Kiev.
Wednesday night saw hundreds of pro-Russian separatists attack a security services base in Mariupol near the Azov Sea.
Troops had responded by opening fire and killing three and wounding 13 of the assailants, Ukraine's Interior Minister Arsen Avakov said.
"They came here around 8:15pm, demanding that we surrender our weapons and join the people. There were some women with them, but then they left," said Police Major Oleksandr Kolesnichenko, deputy commander of the base.
"Then they used a truck to break through the gate. There was some incoming fire. I could not see who was shooting - it was dark," he said.
A separatist representative quoted by Reuters news agency, who gave his name only as Sergei, said there had been a peaceful rally at the base.
"We had a peaceful rally to urge the police to join the people. The commander of the compound warned he would order troops to shoot to kill.
"Then there was shooting. Some people came with Molotov cocktails."
The clash took place hours after apparently unsuccessful attempts by the Ukrainian military operation to retake territory elsewhere in eastern Ukraine from armed pro-Russian rebels.
In one instance, an armoured column of Ukrainian paratroopers lost control of some of their armoured vehicles to pro-Russian separatists.
The United States and European Union have imposed visa bans and asset freezes on a small number of Russian government officials, but say they are now considering more serious measures, accusing Moscow of encouraging anti-government elements in Ukraine.
"What I have said consistently is that each time Russia takes these kinds of steps that are designed to violate their sovereignty, that there are going to be consequences," US President Barack Obama said on Wednesday in an interview.
The EU, which depends on Russia for some 30% of its gas supplies, is currently discussing the possible impact of economic sanctions with member states.