Russia will find out just how isolated it is on the world stage on Monday as the United Nations holds a rare emergency special session of the General Assembly to discuss whether to condemn Moscow's attack on Ukraine.

All 193 members will have their say as the global body decides if it will support a resolution condemning Russia's “unprovoked armed aggression” in Ukraine and demand its immediate withdrawal.

It is only the 11th time in the UN's history that such an emergency session has been held.

And it will be seen as a barometer of democracy in a world where autocratic sentiment has been on the rise, diplomats said, pointing to such regimes in Myanmar, Sudan, Mali, Burkina Faso, Venezuela, Nicaragua — and, of course, Russia.

If Moscow wins in Ukraine, the international order could be “changed forever”, one senior diplomat told AFP on condition of anonymity, underscoring the gravity of the moment at the body charged with global peace and security.

The meeting will begin at 10am in New York with speeches by assembly president Abdulla Shahid and UN Secretary-General Antonio Guterres.

With many countries expected to speak, a vote on the resolution may not come until Tuesday.

Its authors hope they may exceed 100 votes in favour — though countries including Syria, China, Cuba and India are expected to either support Russia or abstain.

Russian President Vladimir Putin launched a full-scale invasion of Ukraine on February 24.

He has deployed tens of thousands of troops in and around the country, equipped with tanks, fighter jets and ships.

Since then Russia has become an international pariah as its forces do battle on the streets of Ukraine's cities, facing a barrage of sanctions including a ban from Western airspace and key financial networks.

On Sunday, Putin ordered Russia's nuclear “deterrence forces” onto high alert, prompting an immediate international outcry, with the United States slamming the order as “totally unacceptable”. Russia has pleaded “self-defence” under Article 51 of the UN Charter.

But that has been roundly rejected by Western countries and the UN, which accuse Moscow of violating Article 2 of the Charter, requiring its members to refrain from the threat or use of force to resolve a crisis.

They were due to repeat those accusations on Monday.

'No ordinary moment'

The move to hold the emergency session was sparked by Russia on Friday using its veto to block a Security Council resolution that condemned Moscow's invasion and called for the immediate withdrawal of its troops.

Russia voted against the resolution, but it did not have veto power to derail the referral of the war to the General Assembly, allowed under a 1950 resolution called “Uniting for Peace”.

It allows for members of the Security Council to seize the General Assembly for a special session if the five permanent members — Russia, the United States, Britain, France and China — fail to agree to act together to maintain peace.

Only the support of nine of the council's 15 members is required to call an emergency special session of the General Assembly.

Eleven countries voted in favour. Russia opposed, while the United Arab Emirates, China and India abstained.

“The council members who supported this resolution recognise that this is no ordinary moment,” said US Ambassador Linda Thomas-Greenfield.

On Monday at 5pm, the Security Council is scheduled to hold an emergency meeting on the humanitarian situation in Ukraine, where up to seven million people are expected to flee the fighting.