Russian-installed leaders in occupied areas of four Ukrainian regions set out plans for referendums on joining Russia, a move Ukraine dismissed on Monday (local time) as a stunt by Moscow to try to reclaim the initiative after crushing losses on the battlefield.

"The Russians can do whatever they want. It will not change anything," Ukraine's Foreign Minister Dmytro Kuleba said in response to reporters' questions at the start of a meeting with US Ambassador to the United Nations Linda Thomas-Greenfield.

In a tweet, he added: "Russia has been and remains an aggressor illegally occupying parts of Ukrainian land. Ukraine has every right to liberate its territories and will keep liberating them whatever Russia has to say."

US National Security Adviser Jake Sullivan said Washington and its allies would reject any such referendums, which he said would bring Russia no benefits on the battlefield.

In the apparently coordinated move, Russian-backed officials announced planned referendums for Sept. 23-27 in the Luhansk, Donetsk, Kherson and Zaporizhzhia provinces, representing around 15 percent of Ukrainian territory or an area about the size of Hungary.

Russia already considers Luhansk and Donetsk, which together make up the Donbas region Moscow partially occupied in 2014, to be independent states. Ukraine and the West consider all parts of Ukraine held by Russian forces to be illegally occupied.

In a post on social media addressed to Putin, the leader of the self-proclaimed Donetsk People's Republic (DPR), Denis Pushilin, wrote: "I ask you, as soon as possible, in the event of a positive decision in the referendum - which we have no doubt about - to consider the DPR becoming a part of Russia."

Some pro-Kremlin figures framed the referendums as an ultimatum to the West to accept Russian territorial gains or face an all out war with a nuclear-armed foe.

"Encroachment onto Russian territory is a crime which allows you to use all the forces of self–defence," Dmitry Medvedev, a former Russian president and now hawkish deputy chairman of Putin's Security Council said on social media.

Margarita Simonyan, editor-in-chief of the pro-Kremlin RT TV station, wrote: "Today a referendum, tomorrow recognition as part of the Russian Federation, the day after tomorrow strikes on Russian territory become a full-fledged war between Ukraine and NATO and Russia, untying Russia's hands in every respect."

Reframing fighting in occupied territory as an attack on Russia could also give Moscow a justification to mobilise its 2 million-strong military reserves. Moscow has so far resisted such a move despite mounting losses in what it calls a limited "special military operation" rather than a war.

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Sullivan said Washington was aware of reports Putin might be considering ordering a mobilisation, which Sullivan said would do nothing to undermine Ukraine's ability to push back Russian aggression.

Kuleba, the Ukrainian foreign minister, also referred to potential Russian mobilisation plans, tweeting: "Sham ‘referendums’ will not change anything. Neither will any hybrid ‘mobilisation’."

'LOUD AND CLEAR'

"As Russia moves ahead with fake referenda in occupied territories of Ukraine, let me say it loud and clear: We will never recognise this. Ukraine has every right to take back its territory," Estonian Prime Minister Kaja Kallas tweeted.

Russia has declared capturing all of Luhansk and Donetsk provinces to be the main aim of its "special military operation" since its invasion forces were defeated in March on the outskirts of Kyiv.

It now holds about 60 percent of Donetsk and had captured nearly all of Luhansk by July after slow advances during months of intense fighting. But those gains are now under threat after Russian forces were driven from neighbouring Kharkiv province this month, losing control of their main supply lines for much of the Donetsk and Luhansk front lines.

The referendums were announced a day after Ukraine said its troops had recaptured a foothold in Luhansk, the village of Bilohorivka, and were preparing to advance across the province.

Ukraine's defence ministry taunted Moscow, which has claimed it abandoned the front in Kharkiv to "regroup" for fighting elsewhere. "Why did the chicken cross the road?" the Ukrainian ministry tweeted. "Because it was regrouping."

Russia controls most of Zaporizhzhia but not its regional capital. In Kherson, where the regional capital is the only major city Russia has so far captured intact since the invasion, Ukraine has launched a major counter-offensive.

Unverified footage on social media showed Ukrainian forces in Bilohorivka, which lies just 10 km (6 miles) west of the city of Lysychansk, which fell to the Russians after weeks of some of the war's most intense fighting in July.

"There will be fighting for every centimetre," the Ukrainian governor of Luhansk, Serhiy Gaidai, wrote on Telegram. "The enemy is preparing their defence. So we will not simply march in."

Pro-Russian officials have said the referendums could be held electronically. Russia staged a referendum in Crimea eight years ago before declaring it annexed. Western countries have dismissed such votes as illegal and fraudulent.

In a move designed to shore up Russia's military in Ukraine, Russia's parliament on Tuesday also approved a bill to toughen punishments for a host of crimes such as desertion, damage to military property and insubordination, if they were committed during military mobilisation or combat situations.