MOSCOW - President Vladimir Putin said on Tuesday that Russia wanted to "save people" in four Moscow-controlled territories in Ukraine, on the last day of annexation votes denounced as a sham by Western leaders.

If the regions vote to join Russia, Moscow will claim sovereignty over about 20 per cent of Ukraine, including Crimea, which was annexed in 2014.

"Saving people in all the territories where this referendum is taking place... is the focus of the attention of our entire society and of the whole country," Mr Putin said during a televised meeting with officials.

The annexation polls organised by Kremlin-installed authorities in four regions of Ukraine mostly controlled by Russian forces are due to close on Tuesday.

Observers say the recent counter-offensive, which saw important successes for the Ukrainian army, pushed Mr Putin to rush ahead with the votes to cement Russia's authority in the occupied territories.

The vote will have security implications, the Kremlin warned on Tuesday, as Moscow threatened to use nuclear weapons and all available means to defend its territory.

Kyiv and its allies said that the West would never recognise the results of the ballot, denounced as a sham.

Elected officials brought ballots door-to-door, in many cases accompanied by armed Russian forces.

Results are expected between Tuesday evening and later this week.

Mr Putin is expected on Friday to formally declare the Ukrainian regions becoming part of Russia, according to Russian news agencies. AFP