The first contingent of US troops has landed in Poland for
military exercises amid tensions with Russia over Ukraine.
An initial 150 soldiers are to be followed by a further 450
within days.
US President Barack Obama has warned Russia it faces new
sanctions if it refuses to implement an agreement to reduce tensions in eastern
Ukraine.
Reports are coming in of violent incidents overnight between
pro-Russian militants and Ukrainian forces in Mariupol and Artemivsk.
Mr Obama accuses Russia of flouting last week's deal on
Ukraine while Moscow has warned it will respond to any attack on its
"interests" in Ukraine.
Speaking on Russian state TV channel RT on Wednesday,
Foreign Minister Sergei Lavrov drew a parallel with the 2008 Georgian war,
saying that if "the interests of Russians have been attacked directly....
I do not see any other way but to respond in full accordance with international
law".
Mr Lavrov also accused the US of "running the
show" in Ukraine, and that it was "quite telling" that Kiev had
re-launched its "anti-terrorist" operation in the east on Tuesday
during a visit by US Vice-President Joe Biden.
US state department spokeswoman Jen Psaki dismissed his
comments as "ludicrous". "Our approach here is de-escalation. We
don't think there's a military solution on the ground," she said.
'Security guarantee'
The 150 soldiers from the 173rd Airborne Brigade arrived in
the Polish town of Swidwin from their base in Vicenza, Italy.
Stephen Mull, the US ambassador to Poland, said the US had a
"solemn obligation in the framework of Nato to reassure Poland of our
security guarantee".
President Obama told a news conference in Japan that Moscow
had failed to halt actions by pro-Russian militants in Ukraine.
The US had further sanctions against Russia "teed
up", he added.
The US troops are expected to be carrying out military
exercises in Poland as well as in Lithuania, Latvia and Estonia for the coming
months.
There has been growing concern in those countries at the
build-up of thousands of troops in Russia along its borders with Ukraine in
recent weeks.
Elsewhere, the UK's Ministry of Defence (MoD) confirmed that
Russian military aircraft had been identified approaching the north of
Scotland, but they turned away shortly after fighter jets were scrambled to
investigate.
Military officials in the Netherlands and Denmark confirmed
they too had scrambled jets to escort the jets away from their airspace.
And in the seas around the UK, a Royal Navy warship is
shadowing a Russian destroyer in what the MoD described as a "well
established and standard response" as it sails past British territory.
The BBC visits Ukrainian soldiers on the border with RussiaBut
the focus of the tension remains eastern Ukraine, where pro-Russian separatists
have taken over administrative buildings in at least a dozen towns in a bid to
seek closer ties to Moscow.
Ukrainian Interior Minister Arsen Avakov announced on
Thursday that the city hall in Mariupol, a port on the Sea of Azov, had been
"liberated" overnight without any casualties.
"Civic activists" played a major part in the
operation, he said.
However, local news website 0629 reported that the
self-proclaimed People's Republic of Donetsk was still claiming control over
the mayor's hall after a struggle with attackers.
Mr Avakov also reported that Ukrainian troops in Artemivsk
had fended off an attempt by dozens of pro-Russian militants to seize weapons
from a military unit. One soldier was wounded, he said.
Unrest began in Ukraine last November over whether the
country should look towards Moscow or the West.