ZAPORIZHZHIA, Sep 28 (Reuters) - Moscow was poised on Wednesday (Sep 28) to annex a swath of Ukraine, releasing what it called vote tallies showing support in four partially occupied provinces to join Russia, after what Kyiv and the West denounced as illegal sham referendums held at gunpoint.

On Moscow's Red Square, a tribune with giant video screens has been set up, with billboards proclaiming "Donetsk, Luhansk, Zaporizhzhia, Kherson - Russia!".

President Vladimir Putin could proclaim the annexation in a speech within days, just over a week since he endorsed the referendums, ordered a military mobilisation at home and threatened to defend Russia with nuclear weapons if necessary.

The Russian-installed administrations of the four Ukrainian provinces on Wednesday formally asked Putin to incorporate them into Russia, which Russian officials have suggested is a formality.

"The results are clear. Welcome home, to Russia!," Dmitry Medvedev, a former president who serves as deputy chairman of Russia's Security Council, said on Telegram.

Russian-backed authorities claim to have carried out the referendums over five days in parts of eastern and southern Ukraine that makes up around 15 per cent of the country's territory.

Residents who escaped to Ukrainian-held areas in recent days have told of people being forced to mark ballots in the street by roving officials at gunpoint. Footage filmed during the exercise showed Russian-installed officials taking ballot boxes from house to house with armed men in tow.

"They can announce anything they want. Nobody voted in the referendum except a few people who switched sides. They went from house to house but nobody came out," said Lyubomir Boyko, 43, from Golo Pristan, a village in Russian-occupied Kherson province.

Russia says voting was voluntary, in line with international law, and that turnout was high. The referendums and notion of annexations has been rejected globally, as was Russia's 2014 takeover of Crimea from Ukraine.

Ukrainian President Volodymyr Zelenskyy sought to rally international support against possible Russian annexations in a series of calls with foreign leaders, including those of Britain, Canada, Germany and Turkey.

"Thank you all for your clear and unequivocal support. Thank you all for understanding our position," Zelenskyy said in a late-night video address.

The United States said it would in coming days impose economic costs on Moscow for the referendums, adding to several tranches of sanctions since Russia's invasion of Ukraine in February that has destroyed cities and killed thousands.

"We will continue to work with allies and partners to bring even more pressure on Russia and the individuals and entities that are helping support its attempted land grab," State Department spokesperson Ned Price told reporters.

The European Union's executive also proposed more sanctions against Russia, but the bloc's 27 member countries will need to overcome their own differences to implement them.