The first shots in Moscow's occupation of Crimea were fired today as Russian troops confronted members of the Ukraine air force. And over the Atlantic, the war of words is heating up.

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Russia's President Putin declared in Moscow today that the use of force in Ukraine is a last resort.

But the first shots in Moscow's occupation of Crimea have already been fired, as Russian troops confronted 200 members of the Ukrainian air force over control of a military base in Crimea.

In remarks to reporters, US President Obama mocked President Putin's justification for sending the Russian military into the Crimean peninsula - something that the US president says is a "violation of international law".

"President Putin seems to have a different set of lawyers making a different set of interpretations," Mr Obama said. "But I don't think that's fooling anybody."

The US president said he held a meeting of the National Security Council on Tuesday morning, his second such session in two days about Ukraine.

The US has promised a loan of $1bn to stabilise the Ukrainian economy and help move the country towards elections. It has also threatened Russia with trade sanctions and the freezing of assets.

Russia responded on Tuesday that there would be a retaliation to such actions.

"We will have to respond," foreign ministry spokesman Alexander Lukashevich said in a statement. "As always in such situations, provoked by rash and irresponsible actions by Washington, we stress: This is not our choice."

'Humanitarian mission'

President Putin told a press conference that Russia is on a "humanitarian mission" in Ukraine, and dismisses threats against his country, such as trade sanctions, as "counterproductive and harmful".

President Putin was giving his first press conference since the ousting of Viktor Yanukovych as Ukrainian president. Mr Putin said there had been an "unconstituional coup d'etat" in Ukraine, and that "in legal terms, there is only one president in Ukraine. It is Yanukovych."

However, President Putin said that he had told Mr Yanukovych that he has "no political future". He added that if the former Ukrainian president had stayed in Ukraine, he would have been "murdered".

All threats against Russia are counterproductive and harmful. Vladimir Putin

The Russian president conceded that the people of Ukraine had clearly wanted "change", but said that change should take place on a "level playing field."

He added that the use of force by Russia was a "last resort", but that Russian retained the right to intervene if there was "lawlessness" in the Russian-speaking east of Ukraine.

"There can be only one assessment of what happened in Kiev, in Ukraine in general," he said. "This was an unconstitutional coup and the
armed seizure of power. No one argues with this. Who can argue with it?

"This is the last resort. We believe, have believed and will believe, that Ukraine is not only our closest neighbour but a brotherly republic."

On Monday various EU and US polticians threatened Russia with "costs" if its intervention in Crimea continued. Discussions with Russia over visas and a G8 conference to be held in Sochi, Russia, were cancelled. The US used stronger language, threatening sanctions and the freezing of assets.

Putin, whose country is a major exporter of gas to the EU, said that leaders should think about the costs they will incur if they carry out such threats.

"All threats against Russia are counterproductive and harmful," he said. Commenting on the decision to cancel the Sochi conference, Mr Putin said "they
don't need to".

Mr Putin also accused the west of using Ukraine for a democratic experiment, like scientists experimenting on lab rats.

"And this is not the first time our Western partners are doing this in Ukraine," he said. "Sometimes I get the impression that across the pond, somewhere in America, staff at some laboratory are sitting there conducting experiments, like on rats, without understanding the consequences of what they are doing."

Shots fired

However, as he spoke, military tensions in the region increased. A stand-off developed between Ukrainian and Russian soldiers at the Belbek airbase in Crimea. The Ukrainians marched to the base, waving flags and singing patriotic songs, and then demanded to be let in.

Russian soldiers responded by firing warning shots and threatening to shoot the Ukrainians in the legs.

Channel 4 News International Editor Lindsey Hilsum is at the scene.

Ukraine's border agency also reported on Tuesday that Russian navy ships were blocking both ends of the Kerch strait, which separates Russia and Crima. Russia has not responded to the claim.

War games cancelled

On Tuesday, President Putin also cancelled military exercises on the border with Ukraine.

Moscow's spokesman told Interfax news agency of the plans for tens of thousands of Russian troops to halt military exercises in western Russia in the early hours of Tuesday morning.

Markets rallied on reports of the pullback after a crash on Monday. However, no immediate confirmation was available that Russian forces had indeed retreated.

"President Vladimir Putin ordered the forces engaged in the military exercises to return to their permanent bases," Interfax quoted Dmitry Peskov, Mr Putin’s press secretary, as saying. He said that the military exercises, which Moscow denied were linked to events in Ukraine, had been a success.

The statement came after Ukraine's defence ministry said its forces in Crimea had received an ultimatum from Russian forces to surrender by 5am local time or face military attack - a claim that was denied by Moscow.

In the end, the deadline passed without incident.

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