Prime Minister Jacinda Ardern has called for a "rallying cry from the world" against Russia for its invasion of Ukraine.

"What you see in Ukraine is illegal, it is immoral. It is causing a loss of civilian life and that loss could extend if, as [Russian President Vladimir] Putin has claimed, he expands the types of weapons used in this war," she said.

She made the remarks at the United Nations in New York where she met with Ukrainian Prime Minister Denys Shmyhal on the sidelines of the United Nations General Assembly yesterday.

Ardern said Putin's decision to partially mobilise additional troops and threat to use additional, possibly nuclear, weapons was an "extraordinary escalation of the conflict" and exemplified the "lie" Putin had told since the start of the conflict: that Russian troops were liberating Ukraine.

Ardern and Shmyhal's informal meeting came about when both were seated next to each other at a food security event.

Tensions escalated between Russia, Ukraine, and the United States today after Putin announced the call-up of 300,000 Russian reservists.

There will also be referendums held in parts of Ukraine over whether they should unify with Russia.

Ardern decried these "sham" referendums, which appear to be designed to give Putin an excuse to annex parts of Ukraine into Russia, escalating the conflict.

Putin also made a veiled threat to use nuclear or other powerful weapons if he felt it necessary.

Ardern said she spoke with Shmyhal about "the ongoing escalation we see in Ukraine".

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She also reaffirmed New Zealand's ongoing support for Ukraine.

Ardern said that everything she had heard from Ukraine was that New Zealand was doing a "significant amount" to assist with its efforts.

"New Zealand has always stood firmly against this illegal invasion, but we stand firmly against escalation of this conflict."

She said the threat of escalation "flies in the face of the lie they have told that are there to liberate others".

United States President Joe Biden delivered his address to the United Nations General Assembly this morning.

In his speech, he decried Russia's escalation of the conflict and veiled threat to use nuclear weapons.

Biden said that Putin made "irresponsible nuclear threats" and repeated a longstanding American position that a "nuclear war can not be won, and must never be fought".

Ardern said the speech reflected "the time we are in".

"Even in the last 24 hours we have seen an extraordinary escalation coming from Putin."

Ardern will make her own speech to the General Assembly later this week. Earlier, she conceded that Ukraine would dominate this week at the United Nations.