US Secretary of State John Kerry warned Russia of "additional consequences" if it fails to pull its troops back from the Ukraine border, as Kiev accused Moscow of aggression in its restive east.
During a telephone call with Russian Foreign Minister Sergei Lavrov on Saturday, Kerry "made clear that if Russia did not take steps to de-escalate in eastern Ukraine and move its troops back from Ukraine's border, there would be additional consequences", a senior State Department official said.
A first wave of US sanctions unveiled in March blacklisted officials and businesspeople close to Russian President Vladimir Putin to protest at Moscow's takeover of Crimea.
US Vice President Joe Biden will travel to Ukraine on April 22 to stress support for Kiev and steps to improve its energy security, the White House said in a statement.
Ukraine accused Moscow of aggression after Kalashnikov-wielding gunmen seized two security buildings in its restive eastern rust belt on Saturday amid spreading protests demanding the Russified region join Kremlin rule.
The coordinated attacks and a series of gunfights between militants and police in two eastern towns underscored the volatility of the crisis ahead of first direct talks between EU and US diplomats and their Moscow and Kiev counterparts in Geneva on Thursday.
"The authorities of Ukraine view today's events as a display of aggression by the Russian Federation," Ukraine's Interior Minister Arsen Avakov wrote on his Facebook page.
Kiev said Ukraine's interim Foreign Minister Andriy Deshchytsya told Lavrov by telephone on Saturday to "stop the provocative activities of Russian special services in the eastern regions of Ukraine."
But Moscow said Lavrov firmly rejected the accusation and "noted that similar claims... have been made by Washington, although we still have not been presented with any concrete proof".
Acting Ukrainian president Oleksandr Turchynov convened an emergency security meeting after his interior minister reported that a "gunfight" had erupted between local security forces and militants who had attacked a police station in the eastern town of Kramatorsk.
The meeting broke up around midnight (2100 GMT) with no announcement except that the participants were considering measures to allow "normalisation of the situation in eastern Ukraine".
Ukraine's interim government has been facing relentless pressure from Russia since its February ouster of an unpopular Kremlin-backed president and decision to seek closer ties with the West.
The seizures highlight how little sway Kiev's untested leaders have over pro-Russians who have since April 6 also controlled the Donetsk government seat and a state security building in the nearby eastern city of Lugansk.
Moscow has massed tens of thousands of troops on Ukraine's eastern border after annexing the Black Sea peninsula of Crimea and nearly doubled the rates it charges Kiev for gas.
Russia is now ready to demand prepayment from the cash-strapped government for future gas deliveries or halt supplies -- a move that would impact at least 18 EU countries and deepen the worst East-West standoff since the Cold War.
A letter obtained by AFP and dated April 11 showed European Commission President Jose Manuel Barroso calling for a common EU response to Putin's latest energy warning.
A note sent by Putin on Thursday cautioning that gas transits through Ukraine may cease due to Kiev's debts to Moscow "raises serious issues for Europe's collective energy security", Barroso wrote.
Barroso said the issue would be raised at a meeting on Monday of EU foreign ministers.
The latest unrest began with morning raids on a police station and local security service centre in Slavyansk -- a riverside town of 100,000 about 60 kilometres (35 miles) north of the regional capital Donetsk.
Ukraine's interior ministry said the first assault was led by 20 "armed men in camouflage fatigues" whose main purpose was to seize 20 machine guns and 400 Makarov guns stored in the police headquarters "and to distribute them to protesters".
An AFP reporter saw the Slavyansk police station surrounded by gunmen in masks and camouflage who had set up a barricade of old tyres and dumpsters in front of the police headquarters.
The interior ministry later reported that its forces had repelled an attack on a Donetsk chemical factory that manufactures explosives.
The police said a separate group of assailants had also unsuccessfully tried to seize the prosecutor's office in Donetsk -- a bustling city of one million that was the seat of power of president Viktor Yanukovych before his ouster and flight to Russia.
But an AFP reporter saw about 200 pro-Russian protesters armed with clubs and sticks storm the city's police headquarters without meeting any resistance.
A few dozen anti-riot police who arrived at the scene were instead sporting orange and black ribbons symbolising support for Russian rule.
The Donetsk administration centre is already being held by gunmen who have proclaimed the creation of their own "people's republic" and called on Putin to send Russian troops into eastern Ukraine.
Ukraine's embattled Prime Minister Arseniy Yatsenyuk promised during an unannounced visit to Donetsk on Friday to grant more powers to the country's regions and protect the east's right to use the Russian language.
But the Donetsk and Lugansk gunmen want to hold independence referendums coinciding with snap presidential polls Ukraine will stage on May 25.
Western leaders and Kiev have accused the Kremlin of orchestrating the unrest in order to justify a possible future invasion of eastern Ukraine -- a charge Moscow flatly denies.
Russia on Friday warned that it would boycott Thursday's Geneva talks should Ukraine try to regain control of the seized buildings through force.