Russia on Wednesday dismissed as "groundless" concerns in Kiev and the West over the presence of its troops near the border with Ukraine, saying they posed no threat, and accused Washington of fuelling tension in the region.

Washington has accused Russia of orchestrating separatist unrest in eastern Ukraine and NATO has urged it to pull back troops from near the border with Ukraine.

"The United States and Ukraine have no reason to be worried," the Russian Foreign Ministry said in a statement. "Russia has stated many times that it is not carrying out any unusual or unplanned activity on its territory near the border with Ukraine that would be of military significance."

The statement also said Moscow had shunned a meeting of the Organization for Security and Cooperation in Europe this week in Vienna which was sought by Washington and Kiev over their concerns about Russia's military presence on the border.

The Russian Foreign Ministry said the United States and Ukraine were conducting "an anti-Russian campaign" at the OSCE and increasing tension over Ukraine, where the authorities are trying to quell unrest by pro-Moscow protesters.

"The everyday activity of Russian troops on its [Russian] territory does not threaten the security of the United States and other OSCE member states," it said. "Attempts to blame Russia for building up its armed forces are groundless."

The Ukrainian government and the U.S. have accused Moscow of fomenting the unrest as a pretext for another Russian military incursion similar to last month's takeover of Crimea. Up to 40,000 Russian troops are massed along the Ukrainian border, according to NATO.

German Chancellor Angela Merkel on Wednesday accused Russia of not taking necessary steps to ease the standoff over Ukraine.

"Unfortunately, in many areas it is not clear that Russia is contributing to a de-escalation of the situation," Merkel said in a speech in parliament.

"Therefore we will continue to do what we've been doing: on the one hand pursue dialogue, but on the other hand make clear that in our view Ukraine has a right to its own development path. We demand this. Ukraine must decide its own future."

Dozens leave offices seized by Ukraine separatists

The situation in eastern Ukraine where pro-Russia separatists are holding state buildings in at least two cities will be solved in the next 48 hours and force will be used if negotiations fail, the interior minister said on Wednesday.

"There are two options - political and negotiations, and force," the minister Arsen Avakov told journalists on the margins of a government meeting.

"For those who want dialogue, we propose talks and a political solution. For the minority who want conflict they will get a forceful answer from the Ukrainian authorities," he said.

Avakov was speaking as anti-government protesters in the eastern city of Luhansk erected high barricades along a thoroughfare running in front of the security service premises.

The Luhansk security services building was among several government offices seized by pro-Moscow groups Sunday in an escalation of protests against the interim government in power since the ouster of President Viktor Yanukovych in February.

Ukraine's Security Service said Wednesday that 56 people held inside the agency's local headquarters in Luhansk by pro-Russian separatists have been allowed to leave.

On Tuesday, security services said separatists inside the building, armed with explosives and other weapons, were holding 60 people hostage. It was not immediately clear if the 56 allowed to leave were among that number, or how many people were still being held.

Those occupying the building have issued a video statement warning that any attempt to storm the place would be met with force.

In the video, posted by Ukrainian media, a masked man identified the occupiers as Ukrainian veterans of the Soviet war in Afghanistan and warned the authorities against trying to retake the building. "Welcome to hell, then!" he said.

The security services said negotiations with the separatists were continuing and that parliamentary deputies had been able to enter and leave the building unhindered.

Overnight, speakers at a gathering in front of the building condemned the government in Kiev and renewed demands to be allowed to hold a referendum on declaring autonomy for their region. That demand is similar to one that preceded Crimea's annexation by Russia.

Speeches at the rally were occasionally interspersed with chants of "Russia, Russia!" and an unidentified speaker listed names of prominent politicians that he suggested should be executed, eliciting cheers in return.

All the cities affected by the uprisings are in Ukraine's industrial heartland in the east, which has a large population of ethnic Russians and economic and cultural ties to Russia are strong. Many residents are suspicious of government that took power in February.