Ukraine crisis: US troops land in Poland for exercises
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.
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.
But 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.
Unverified footage of military helicopters, said to be flying over Artemivsk, was posted by a blogger on YouTube.
Unrest began in Ukraine last November over whether the country should look towards Moscow or the West.