Image copyright AP Image caption The US and Nato have recently strengthened their military presence in eastern Europe

Moscow has voiced concern over an "unprecedented" increase in US and Nato military activity near Russian borders, amid an escalating crisis in Ukraine.

Russia's defence minister condemned "provocative" US and Nato comments.

Pro-Russia activists have seized buildings in more than a dozen towns in east Ukraine and hold seven European military observers in Sloviansk.

The EU has stepped up its sanctions, naming 15 new targets on Tuesday, a day after the US ordered similar measures.

Analysis Russia's claims of an "unprecedented" Nato military build-up on its borders are part of an ongoing information war. Nato insists that its deployments are simply to reassure its worried members. Company-size groups of around 150 US paratroops are being sent for exercises in Poland and the three Baltic republics. So that's about 600 men. Around a dozen additional fast jets have been deployed to Poland and to reinforce air patrols in the Baltic states' airspace. Latvia, Estonia and Lithuania - all Nato members - have no jet fighters of their own. And to cap Nato's "build-up", a small flotilla of mine hunters - almost as small a vessel as you can get in naval terms - has been sent to the Baltic Sea. The US has reinforced its naval presence in the Black Sea, but again this is a question of showing presence. By contrast Russia has some 40,000 armoured and mechanised troops on Ukraine's border, which Nato says are ready to advance at short notice.

The US sanctions targeted seven Russian individuals and 17 companies which Washington says are linked to President Vladimir Putin's "inner circle".

The European Union's list includes Gen Valery Gerasimov, chief of the Russian General Staff, and Lt Gen Igor Sergun, identified as the head of the Russian military intelligence agency, the GRU.

It also includes Russian Deputy Prime Minister Dmitry Kozak and pro-Russian separatist leaders in Crimea and in the eastern Ukrainian cities of Luhansk and Donetsk.

The BBC's Matthew Price in Brussels says this list does not appear to follow the US line in targeting President Putin's associates, but rather those involved in events on the ground in Ukraine.

'Provocative'

In a statement, Russian Defence Minister Sergei Shoigu said he had a "candid" hour-long phone call with his American counterpart Chuck Hagel.

Mr Shoigu stressed that US and Nato military activity in eastern Europe was accompanied by "provocative" statements about the need to "contain" Russia.

Media playback is unsupported on your device Media caption Sarah Rainsford saw pro-Russians clash with a group marching for a united Ukraine

The US has sent 600 troops to Poland and the Baltic states. Washington says it has deployed the extra troops to reassure Nato allies.

Image copyright AFP Image caption Pro-Russian activists continued to occupy government buildings in Sloviansk on Tuesday

Image copyright AP Image caption The separatists seized a local government building in Kostyantynivka on Monday

Image copyright Reuters Image caption A number of people were injured in clashes in the eastern city of Donetsk on Monday

Mr Shoigu announced that Russian troops had returned to their "permanent positions" after conducting military exercises on the border with Ukraine.

The Pentagon said Mr Shoigu had given "assurances that Moscow has no plans to invade Ukraine".

Deputy Foreign Minister Sergei Ryabkov repeated the line on Tuesday, telling Interfax that Moscow was "not at all inclined to repeat the so-called Crimea scenario in south-eastern Ukraine. There are no grounds to fear this".

Image copyright Reuters Image caption Among the sanctions targets is Igor Sechin, who has worked for Vladimir Putin since the early 1990s

Mr Hagel called on Moscow to help secure the release of the seven military observers linked to the Organization for Security and Co-operation in Europe who were seized by pro-Russian gunmen in Sloviansk last week.

Some 40 people, including journalists, pro-Kiev activists and three members of Ukraine's security service are being held there.

The Russian ambassador to the OSCE, Andrei Kelin, earlier said Moscow was taking "steps" to secure the observers' release.

Journalist's ordeal

The US and EU first imposed visa bans and asset freezes on a number of senior Russian officials and companies after Moscow annexed Crimea from Ukraine last month.

Media playback is unsupported on your device Media caption US journalist Simon Ostrovsky tells of his ordeal at the hands of pro-Russian abductors

On Monday, Washington added to its sanctions list "in response to Russia's continued illegal intervention in Ukraine".

Russian Deputy Foreign Minister Sergei Ryabkov said Moscow's response would be "painful for Washington".

Among the individuals named were Igor Sechin, head of state oil giant Rosneft, and Alexei Pushkov, chairman of the committee of international affairs of Russia's lower house.

Gas producer Gazprom, whose chief executive was spared sanctions, warned in a statement on Tuesday that further measures could damage its business and the BBC's Daniel Sandford in Moscow says there is a sense of nervousness in Russia that sanctions may start to bite.

The EU has now announced sanctions on a total of 48 individuals. It has avoided targeting business leaders such as Mr Sechin.

The US has accused Russia of "doing nothing to meet the commitments it made" at a meeting with Ukraine, the US and EU in Geneva on 17 April, which it said had included refraining from violence or provocative acts.

Name Position Sanctioned by Putin's 'inner circle' Gennady Timchenko Founder of Gunvor (oil and energy market trading) US Arkady Rotenberg and Boris Rotenberg Co-owners of SMP Bank and SGM Group US Yuri Kovalchuk Largest single shareholder of Bank Rossiya US Igor Sechin Head of Rosneft (petroleum company) US Government officials Sergei Ivanov Chief of staff for Presidential Executive Office US Oleg Belaventsev Russian presidential envoy to Crimea US and EU Vladimir Yakunin Chairman of Russian Railways US Igor Sergun Director of GRU EU Valery Gerasimov Chief of General Staff of Russian Armed Forces EU Vladimir Kozhin Head of administration US Viktor Ivanov Director of Federal Drug Control Service US Sergei Naryshkin Speaker of the lower house of parliament US and EU Vladislav Surkov Presidential aide and election adviser US and EU Dmitry Rogozin Deputy Prime Minister US and EU Sergei Glazyev Adviser on Ukraine policy US and EU Sergei Mironov Member of Russian Parliament US Dmitry Kozak Deputy Prime Minister US and EU Ludmila Shvetsova Deputy Chair State Duma EU Sergei Chemezov Director of Rostec (state high-technologies division) US Others Bank Rossiya Russian bank US Dmitry Kiselyov State television news anchor EU

Meanwhile, a US journalist who was kidnapped and held hostage for several days last week by pro-Russian activists has been speaking to the BBC.

Simon Ostrovsky said he was pulled out of his car at a checkpoint in Sloviansk.

"I was separated from my other colleagues and taken down into the basement, blindfolded. I had my hands tied behind my back. I was thrown on the floor and beaten up and held there for the next three days," he said.

But he said he was unable to confirm any of those involved were from Russia.