It followed his announcement on state TV that the eastern Ukrainian regions of Donetsk and Luhansk were independent.

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Analysts had feared it could be the start of a fully-fledged Russian invasion of the country. It now appears that may be underway.

Here Sky News looks at Donetsk and Luhansk in detail and why they are at the heart of Vladimir Putin's tensions with the West.

Why are Donetsk and Luhansk disputed?

In 2014, violent protests in Ukraine led to the ousting of its pro-Russian leader Viktor Yushchenko.

In response Russia illegally annexed the Ukrainian region of Crimea, which caused international outrage and reignited tensions between the Kremlin and the West.

After the annexation, protests against the new Ukrainian government by pro-Russian separatists started to break out in the Donbas region of eastern Ukraine.

Russia supported the separatist demonstrations as a means of attacking the Ukrainian government.

Fighting saw the separatists establish a People's Republic of Donetsk and People's Republic of Luhansk inside the two areas or 'oblasts' of the same name, which together make up the wider Donbas region.

Professor Michael Clarke, former director general of the Royal United Services Institute (RUSI) and security analyst, explains: "Those breakaway areas claim the whole of Donetsk and Luhansk, but they physically only control around a third of them."

Neither people's republic is recognised by any country apart from Russia, which this week recognised them as independent from Ukraine and signed friendship treaties with both of them.

The Ukrainian government in Kyiv has no direct communication with their leaders and refers to the breakaway regions as "temporarily occupied territories", similar to Crimea.

Fighting in the region has resulted in around 14,000 deaths since 2014.

Some 29 ceasefires, including two key ones brokered by France and Germany in 2014 and 2015, known as the Minsk Accords, have been put in place since the conflict began - but have been consistently broken.

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Where is the frontline?

The frontline of the conflict between the Ukrainians and pro-Russian separatists is the 260-mile border between the so-called People's Republics of Donetsk and Luhansk and the surrounding regions of Donetsk and Luhansk not under rebel control.

This border is known as the Line of Contact or Line of Control.

Those living in the towns and villages along that border have been the main victims of the conflict, with Russian forces carrying out shelling on a regular basis.

This week unmarked tanks have been seen on the outskirts of Donetsk, prompting remarks by several UK politicians that a Russian invasion of Ukraine has begun.

Professor Clarke says: "Since 2014, this has been used by the Kremlin as a frozen conflict, which rumbles on, but which Putin can decide to boil up or down as he pleases.

"Essentially, he's had it in his pocket as a crisis he can create if he wants to."

He adds that while Ukraine has been "restrained" in its military response to Russian forces in recent months - to try to avoid an invasion in the east - a crossover at the border would be a "huge step" and Ukrainian forces "would have to fight".

What is life like inside the Donetsk and Luhansk?

Russian speakers in eastern Ukraine date back to when workers from the former Soviet Union were sent there after the Second World War.

The Donbas region in its entirety stretches across 6,500 square miles and is fairly densely populated compared to western Ukraine - with around four million people in total.

Before the conflict it was mostly known for being an industrial, coal mining area.

But the fighting has seen more than a million people flee the separatist regions.

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Describing life for those left, Professor Clarke says: "The standard of living there is quite poor. It's become quite run down and the war over the last eight years hasn't helped.

"Many of them are people who were too old or vulnerable to leave their homes when it all started in 2014."

Do people living there feel Ukrainian or Russian?

Although some are Russian speakers, pro-Russian sentiment is not as strong in the Donbas region as it is in Crimea.

"They are not strongly nationalist," Professor Clarke says.

"The evidence is that they would welcome being backed by Russia in economic terms, but the Kremlin may well be overestimating the support it has there.

"The leaders are vehemently pro-Russian, but the population is a bit wearier, because they don't want any more fighting - and intelligence on the ground suggests that the idea of them being used by the Kremlin is also coming through."

However formal opposition inside the disputed regions is virtually non-existent, he adds.

"According to Western intelligence, the people who don't like what's happening in the republics are too afraid to speak out."

Who are the separatist leaders?

The Donetsk and Luhansk breakaway areas have their own self-proclaimed presidents.

Denis Pushilin is head of the People's Republic of Donetsk and Leonid Pasechnik leads the People's Republic of Luhansk.

Both have strong Russian links and were formally invited to be part of Russia's ruling United Russia party in December last year.

Pushilin was behind a major Russian ponzi scheme and survived two assassination attempts in June 2014.

Pasechnik's family lived in Russia's far east and previously worked for the Ukrainian Security Service in Luhansk.

Professor Clarke described them as "puppets" for the Kremlin.

"They're businessmen. They're politicians only in the sense of mafia politics," he says.

"They're not important to the Russians. They're just puppets who they tell what to do."

Why is Putin's 'recognition' of Donetsk and Luhansk so important?

Analysts claim that Putin's formal recognition of the two regions as independent marks the start of a wider invasion in Ukraine, which could result in a war between a Western-backed Ukraine and Russia.

Professor Clarke says: "It's important because it's completely illegal under international law.

"And it's another step on the road Putin started in 2014. Effectively he's half-way to formally annexing those two territories as he did with Crimea."

He predicts that should Russian forces cross the Line of Contact and invade the greater Donetsk and Luhansk regions, fighting will break out in the cities of Sloviansk and Sievierodonetsk.

"If Putin wants to push further into Ukraine, we could see local fighting in one or both of those cities, as they are both quite pro-Russia," he says.

"An uprising by Russian speakers there would provide Putin with the justification for further conflict."

Western intelligence sources claim that an order for a "complete invasion" of Ukraine has already been given. This would mean invasions of several major Ukrainian cities and ultimately a coup of the government in Kyiv.

"If tanks go over the Line of Contact, it's a huge step, because Putin will then be seeking to conquer areas that aren't Russian."

With the UK, US, Germany and other nations imposing harsh sanctions on the Kremlin to deter it from war, Putin could still decide to withdraw, Professor Clarke adds, but this is relatively unlikely.