Canadian Foreign Affairs Minister John Baird announced additional sanctions Saturday in response to the escalating tensions in Ukraine, accusing Russia of "violating its sovereignty and territorial integrity."

"We are imposing sanctions on two additional individuals and on a Crimean oil and gas company," said a statement released by Baird.

"[We] will continue to work with allies and like-minded countries to apply pressure to Russia until it de-escalates the situation in Ukraine."

Also on Saturday, the White House announced U.S. Vice President Joe Biden will travel to Kyiv on April 22 to meet with government officials and civil society groups ahead of the Ukrainian presidential election in May.

The announcement came soon after Russia warned the United States that any armed action by Ukrainian authorities in the east of Ukraine would undermine efforts at a diplomatic solution to the conflict and put planned peace talks at risk.

Russian Foreign Minister Sergei Lavrov made the warning during a call from U.S. Secretary of State John Kerry, who expressed concern about Russia's role in "inciting" trouble in eastern Ukraine, the Russian Foreign Ministry said in a statement. Separatists have seized state premises in eastern Ukrainian towns in recent days.

Lavrov said Ukraine was "demonstrating its inability to take responsibility for the fate of the country".

He said that any use of force against Russian speakers in the east of Ukraine "would undermine the potential for cooperation ... including the holding of planned four-party talks in Geneva" on April 17 between Russia, Ukraine, the United States and the European Union.

Ukraine condemned attacks by pro-Russian separatists in eastern Ukraine on Saturday as an "act of aggression by Russia," Interior Minister Arsen Avakov said.

"Units of the interior and defence ministries are implementing an operational response plan," he said in a statement on his Facebook page.

Armed men seized control of police stations and state security headquarters in multiple eastern Ukrainian cities on Saturday. 

Handguns, weapons seized

At least 20 men armed with pistols and rifles took over the police and security services buildings in Slaviansk, a city of 120,000 about 150 kilometres from the border with Russia in the Donetsk region of Ukraine. 

They wore St. George's ribbons, which have become a symbol of pro-Russian protesters in eastern Ukraine. The ribbons were originally associated with the Soviet Union's victory in World War II.

The militants allegedly seized at least 400 handguns and 40 automatic weapons from the police station and replaced the Ukrainian flag with the Russian flag before moving on to a security building in the same city. 

"The aim of the takeover was the guns," a police statement issued from the capital Kyiv said. "They are giving these guns to participants in the protest in Slaviansk."

Ukraine crisis

Pro-Russian protesters helped armed, masked militants build barricades of tires in anticipation of possible police action against the seized locations. (Gleb Garanich/Reuters)

Local sympathizers brought tires to the police station to start building barricades.

The same group of militants began setting up checkpoints on major roads into the city on Saturday evening.

Ukrainian Interior Minister Arsen Avakov pledged a "very tough response" to the seizure while local media reported special forces dispatched to the area. 

A masked guard in Slaviansk, who gave his name only as Sergei, told The Associated Press they have "only one demand: a referendum and joining Russia."

The man said they seized the building because they wanted to protect it from radical nationalists from western Ukraine and "the junta who seized power in Kyiv."

"We don't want to be slaves of America and the West," he said. "We want to live with Russia."

Turmoil in Donetsk

Only hours later, men in the uniforms of Ukraine's now-defunct Berkut riot police occupied police headquarters in the city of Donetsk proper. 

The regional police chief resigned shortly after the Ukrainian flag flying above the station was replaced with a Russian separatist flag. 

"In accordance with your demands I am stepping down," police chief Kostyantyn Pozhydayev told protesters.

The occupation is a potential flash point because if the militants are killed or hurt by Ukrainian forces, that could prompt the Kremlin to intervene to protect the local Russian-speaking population, a repeat of the scenario in the Crimea region when Russian troops were sent in.

Ukraine-protesters-seize-bldgs-apr-7

Protesters, who have also held an administration building in Donetsk since Sunday, initially called for a referendum on secession but later reduced the demand to a vote on autonomy within Ukraine with the possibility of holding another later on whether to join Russia.

On Saturday evening, acting Ukrainian President Olexander Turchynov called an emergency meeting of the country's national security council to deal with the escalating tensions in the east. 

Foreign ministers speak

Ukraine's foreign minister urged Russia to end what he called "provocative actions" by its agents in eastern Ukraine.

Acting foreign minister Andrii Deshchytsia said he had spoken in a phone call with Russian Foreign Minister Sergei Lavrov on Saturday. 

Russia denies providing any support to the militants, who have seized four government buildings in the east of the former Soviet republic, apparently emboldened by Russia's annexation of Ukraine's Crimea region last month.

Russia's Foreign Ministry on Friday warned the Ukrainian government against using force against protesters, saying that such action would derail the talks on settling the crisis between the U.S., the EU, Russia and Ukraine set for next week, as well as any other diplomatic efforts.

In eastern Ukraine, government buildings in the cities of Donetsk and Luhansk have been occupied by pro-Russian protesters who want their regions to split from Kyiv.

On Friday, a deadline set by the Kyiv authorities for the protesters to end their occupation expired, but there was no sign of action from the Ukrainian police to force them out.

With files from The Associated Press, CBC News