New Zealand has joined its Western allies in not recognising results of referenda in Russia-occupied regions of Ukraine and labelling them a "sham".

Russia has said it would formally annex four regions parts of occupied Ukraine where Kremlin-orchestrated "referendums" were held on living under Moscow's rule.

Moscow claimed residents overwhelming supported for their areas to formally become part of Russia.

The Ukrainian government and the West have denounced the referenda as illegal and rigged.

Foreign Minister Nanaia Mahuta said Aotearoa New Zealand did not recognise the results of the "sham referenda in Russia-occupied regions of Ukraine".

"These so-called referenda were not free or fair, and they very clearly were not held in accordance with democratic principles," Mahuta said.

"Instead, they were hastily organised under the threat of force, and Aotearoa New Zealand does not recognise the results."

The referenda come as Russia is accused of further escalating its war on Ukraine. Last week, President Vladimir Putin announced the mobilisation of 300,000 reservists and made what have been interpreted as veiled references to the country's nuclear arsenal.

Mahuta said there was only one course of action available to Russia.

"Cease immediately its illegal invasion of Ukraine, withdraw its troops, and return to diplomatic negotiations to resolve this conflict.

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"Aotearoa New Zealand remains in solidarity with Ukraine in defence of its sovereignty and territorial integrity, and the rules-based international system.

"This war has inflicted immense suffering on Ukraine and displaced millions of its civilians.

"Aotearoa New Zealand also stands firmly against the escalation of Russia's invasion of Ukraine, including mobilisation of Russian citizens.

"Escalation flies in the face of Russia's lie that they are there to liberate Ukraine."

The United States and Western allies earlier condemned the referenda as "sham" and vowed never to recognise their results.

German Foreign Minister Annalena Baerbock on Thursday joined other Western officials in denouncing the votes.

"Under threats and sometimes even [at] gunpoint people are being taken out of their homes or workplaces to vote in glass ballot boxes," she said at a conference in Berlin.

"This is the opposite of free and fair elections," Baerbock said. "And this is the opposite of peace. It's dictated peace. As long as this Russian diktat prevails in the occupied territories of Ukraine, no citizen is safe. No citizen is free."

This week the Government added another 19 individuals to its sanctions regime over Russia's invasion of Ukraine.

The Russia Sanctions Act allows the Government to freeze assets, instate travel bans or prohibit financial dealings. To date, New Zealand has imposed sanctions on more than a thousand individuals and entities.

- with AP