Ukraine's army is probing rebel defenses around the besieged eastern town of Slavyansk, where a long stalemate is under way in what has become a crucible for the worsening crisis.

Three outlying checkpoints set up by insurgents around the town of 160,000 inhabitants were "liquidated" overnight with no casualties on either side, the interior ministry in Kiev said in a statement.

President Vladimir Putin will make his first visit to the Crimean peninsula since Russia annexed the territory in March, reports said Wednesday, in a move to bolster public support amid simmering tensions in Ukraine. 

Russian dailies Kommersant and Gazeta.ru reported Putin could attend a May 9 military parade marking victory in World War II in Sevastopol, which hosts the Black Sea fleet.

The International Monetary Fund said on Wednesday that Russia is already in recession and slashed its growth forecast for 2014 citing the effect of the Ukraine crisis on investment.

"If we define recession as negative growth in two quarters in a row, then Russia from that point of view is experiencing recession," IMF economist Antonio Spilimbergo was quoted as saying by the Interfax news agency.

Pro-Russian insurgents Wednesday lifted an armed siege of a regional police headquarters in the eastern Ukrainian city of Lugansk after the chief officer agreed to resign, as Kiev said its army is on "full combat alert" against a possible Russian invasion.

A police spokeswoman said the officers inside had refused to give up their weapons to a crowd of some 1,000 pro-Moscow militants led by 30 or so armed men carrying Kalashnikov assault rifles and grenade-launchers, who had tried to storm the building overnight.

Russia and the United States stepped up their rhetoric over the spiraling crisis in Ukraine, as pro-Moscow militants shored up control of key buildings in the country's increasingly chaotic east Wednesday.

President Vladimir Putin threatened that U.S. sanctions against Moscow could harm Western energy interests in Russia, which the West blames for stoking the worst confrontation since the end of the Cold War.

Ten soldiers and civilian staff at a Russian military munitions depot burnt to death while fleeing a fire that sparked a series of explosions, the defense ministry said Wednesday.

The fire broke out Tuesday evening at the storage facility at a military base close to the village of Bolshaya Tura in eastern Siberia. Rescue workers only discovered the victims' bodies the next morning.

Russian ambassador to Lebanon Alexander Zasypkin stressed on Wednesday the importance of consensus among the rival parties to elect a new head of state according to the constitution and within the deadline.

“Foreign countries should encourage the Lebanese to carry out (the elections) without interfering in the details,” Zasypkin said in an interview published in As Safir newspaper.

More than 3,000 pro-Russian protesters stormed the regional administration building in the eastern Ukraine city of Lugansk on Tuesday, as Washington described recent violence in the region as “terrorism.”

A vanguard of around 20 youths armed with metal bars broke a window to get inside the building, which was not protected by police.

If Russia invades Ukraine, "they don't need tanks, they can just walk," smiles Lev Nikolaevich, pointing with his deformed finger to the Russian border just behind the trees. "And we will welcome them."

While the world frets about an invasion amid the worst East-West confrontation since the Cold War, the 90 or so people in the tiny border village of Hryhorivka are more relaxed.

A team of IAEA experts is expected to visit two of Iran's nuclear sites within the next week, as part of a monitoring process agreed with the U.N. agency.

Iran's official IRNA news agency reported Tuesday that inspectors would travel to the Ardakan yellow-cake production plant and the Saghand uranium mine, located close together around 450 kilometres (280 miles) from Tehran.