Ukraine crisis: UK RAF fighter jets scrambled to investigate Russian planes as US troops land in Poland

First contingent of soldiers are deployed as President Obama warns Moscow a new round of economic sanctions are being 'teed up'

RAF fighter jets have been scrambled to investigate Russian planes which flew close to UK airspace, after 150 US troops landed in Poland to begin military exercises amid growing tensions with Russia over the escalating Ukraine crisis.

Officials said Typhoon aircraft were dispatched from Scottish base RAF Leuchars after the Russian presence was detected on Wednesday. The jets are believed to have turned away shortly afterwards, the Ministry of Defence said.

The Netherlands and Denmark also scrambled jets after Russian military aircraft approached their airspace.

A MoD defence spokesman said: "Typhoon quick reaction alert aircraft were launched from RAF Leuchars to determine the identity of unknown aircraft that approached the Nato air policing area north of Scotland and could not be identified by other means.

"The aircraft were subsequently identified as Russian military aircraft. The Russian military aircraft remained in international airspace at all times as they are perfectly entitled to do so.”

The first contingent of US troops will be joined by a further 450 within the next few days, as the Ukrainian government said it repelled an overnight raid on a base at Artemivsk, between Donetsk and Slaviansk, in eastern Ukraine on Thursday.

Interior Minister Arsen Avakov said on Facebook the attacks were led by pro-Russian soldiers.

Mr Avakov also said police have cleared the city hall in Mariupol which had been occupied by pro-Russian protesters for more than a week, adding that no casualties have been reported.

Ukrainian troops with five light armoured vehicles also took control of a checkpoint north of Slaviansk on Thursday after pro-Russian separatists appeared to abandon the position, witnesses at the scene said.

On Wednesday, the Russian Foreign Minister Sergey Lavrov compared the current situation to circumstances that triggered the war with Georgia in 2008 and warned any attacks on Russian citizens would invite a response.

His comments came day after Ukraine’s interim president Oleksandr Turchynov announced it was re-launching an anti-terrorist operation against pro-Russia insurgents occupying government facilities in the mostly Russian-speaking east.

Speaking on state TV, Mr Lavrov said: “[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".

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Meanwhile, President Barack Obama cautioned on Thursday that a new round of economic sanctions are 'teed up' after accusing Moscow of failing to live up to an agreement made at Geneva last week to ease tensions in eastern Ukraine.

Mr Obama said the United States needs to secure the support of allies to ensure that additional economic pressure can be applied in response to Russia's aggressive moves on Ukraine's eastern border and the growing unrest in the country driven by pro-Russian seperatists.

He did not put a timeline on when sanctions could be applied, saying only it was a matter of days.

"There was some possibility that Russia could take the wiser course after the meeting in Geneva," Mr Obama said during a press conference with Japanese Prime Minister Shinzo Abe. "So far at least, we have seen them not abide by the spirit or the letter of the agreement."

“There are some things the United States can do alone but ultimately it's going to have to be a joint effort, a collective effort.

“It's important to emphasise that throughout this process our goal has been to change Mr. Putin's calculus, that our preference is to resolve this diplomatically, that sanctions hurt Russia more than anybody else but they are disruptive to the global economy.”

Additional reporting by agencies