President Vladimir Putin has explicitly given his support to the referendums in Russian-occupied Ukraine. 

It's not the first time Moscow has seized and then integrated parts of Ukraine into its own borders: it used similar tactics in 2014 to claim the Crimean peninsula. But on this occasion, there is concern that further steps may increase the likelihood of a nuclear attack. 

Here's why. 

Which areas of Ukraine are the referendums taking place?

In an apparently coordinated move, pro-Russian regional leaders in Ukraine on Tuesday announced referendums to be held from September 23 to 27.

These votes are taking place in the eastern Luhansk and Donetsk regions that make up the Donbas region, Ukraine's industrial heartland, as well as in the southern Kherson and Zaporizhzhia provinces. Altogether, this represents around 15 percent of Ukrainian territory.

READ MORE 

Leaders celebrate the life of Queen Elizabeth

Fashion icon, war child, mother - life of a Queen

Chinese student comes to soprano's rescue

Pro-Russian separatists and Russian troops had already seized large areas of the Donbas in 2014, which later became the self-styled Donetsk (DPR) and the Luhansk People's Republics (LPR). Moscow recognized these areas as independent states just before launching its attack in February, despite Ukraine and the West considering all parts of Ukraine held by Russian forces to be illegally occupied.

Currently, Russia holds about 60 percent of Donetsk and had captured nearly all of Luhansk by July after months of intense fighting.

However, those gains are currently under threat after Russian forces were driven from the neighboring Kharkiv province this month, which has forced the Kremlin took act swiftly to cement its control over the territory it still has. 

Why does Russia want these areas?

On Wednesday Putin announced that 300,000 reservists would be drafted to fight in Ukraine - a major escalation of the conflict - but he also used the opportunity to restate Russia's aims of its "military operation" - to "liberate" the Donbas and other Ukrainian regions.

This ultimately means absorbing the predominantly Russian-speaking areas, where ethnic Russians make up a significant part of the population and sympathy for Moscow and Ukraine's pro-Russian parties has historically been strongest.

There are obvious economic advantages for Russia in integrating a heavily industrialized, resource abundant region into its economy. Such a seizure would give Moscow a large military buffer with the West and also enable it to create a "land bridge" to Crimea, which has become an important maritime hub for Russia. 

However, Western critics are interpreting Putin's recent support as a knee-jerk reaction to heavy Russian losses in northeastern Ukraine, stating that Moscow wants to shore up its gains before Ukraine has a chance to launch similar counter-offensives in the south.

Conversely, the two reasons the president cited in his national address for supporting the referendums were wanting self-determination for the regions' people, and the apparent desire to protect pro-Russians from reprisals by the Ukrainian state.

"We will support the decision on their future, which will be made by the majority of residents in the Donetsk and Luhansk People's Republics, Zaporizhzhia and Kherson," Putin said. "We cannot, have no moral right to hand over people close to us to the executioners, we cannot but respond to their sincere desire to determine their own fate."

What's the precedent and how have Kyiv and the West reacted?

When pro-Russian forces occupied the predominantly Russian ethnic Crimean peninsula in 2014, a similar referendum was held on joining Russia on March 16. Crimea's Russian-backed leaders declared a 97 percent vote to secede from Ukraine, and Russia formally recognized the addition of Crimea to its territory just five days later. 

Kyiv and the West have refused to recognize this, accusing Moscow of rigging the election. 

They have taken much the same tone this time around, calling the referendums propoganda and an illegal sham and vowing never to accept its results.

Ukrainian President Volodmyr Zelenskyy brushed off the plans, saying they were a "sham" that would not be recognized by most countries. "We will act according to our plans step by step. I'm sure we will liberate our territory," he said.

Foreign Minister Dmytro Kuleba echoed previous statements from Kyiv that Ukraine would not rest until every Russian soldier was ejected from its territory: "The Russians can do whatever they want. It will not change anything."

The White House called the votes "an affront to the principles of sovereignty", while EU foreign policy chief Josep Borrell said the referendums were illegal and counter measures would be considered. 

France's President Emmanuel Macron described the plans as a parody. "If the Donbas referendum idea wasn't so tragic, it would be funny," he told reporters in New York.

How could it increase the threat of nuclear attacks? 

If Moscow formally claimed a vast additional chunk of Ukraine, Putin would essentially be daring the U.S. and its European allies to risk a direct military confrontation with Russia, the world's biggest nuclear power.

"All this talk about immediate referendums is an absolutely unequivocal ultimatum from Russia to Ukraine and the West," said Tatiana Stanovaya, founder of the political analysis firm R.Politik.

Former Russian President and staunch Putin ally Dmitry Medvedev said the move would give the Kremlin more options for the defense of what he said would become Russian territory.

"Encroachment onto Russian territory is a crime which allows you to use all the forces of self–defense," Medvedev said in a post on Telegram. "This is why these referendums are so feared in Kyiv and the West."

Russia's military doctrine allows the use of nuclear weapons if weapons of mass destruction are used against it or if the Russian state faces an existential threat from conventional weapons.

U.S. President Joe Biden warned in March that a direct confrontation between the NATO military alliance and Russia would mean World War III. Biden and NATO leaders have been careful to say that they do not want NATO troops in direct conflict with Russian troops.

Putin and senior Russian generals and officials, though, already cast the conflict as a broader contest with the West which they say has sent Ukraine advanced weaponry and helps guide Ukrainian forces with intelligence and training that ultimately kills Russian troops.