Russian Foreign Minister Sergei Lavrov responded with defiance, defending Russia’s record and accusing Ukraine and its allies of threatening his country, before walking out at the end of his speech to the Security Council.

The diplomatic confrontations came a day after Russian President Vladimir Putin launched a new mobilisation campaign for the seven-month-old war that has already killed thousands, displaced millions, pulverised cities and damaged the global economy.

He also effectively announced plans to annex four Ukrainian provinces – saying the government would assist with referendums that are due to start on Friday on joining them to Russia – and threatened to use nuclear weapons.

UN Secretary-General Antonio Guterres told the Security Council that talk of a nuclear conflict was “totally unacceptable” and said any move to annex territory would break international law.

“I am also deeply concerned by reports of plans to organise so-called ‘referenda’ in areas of Ukraine that are currently not under government control,” Guterres said.

“The very international order we’ve gathered here to uphold is being shredded before our eyes,” US Secretary of State Antony Blinken told the gathering.

“We can’t let President Putin get away with it.”

In his speech, Lavrov accused Ukraine of “brazenly trampling” the rights of Russians and Russian-speakers in Ukraine.

“I can assure you that we will never accept this,” he said.

“Everything I’ve said today simply confirms that the decision to conduct the special military operation was inevitable.”

It was at least the Security Council’s 20th meeting on Ukraine this year.

The body has been unable to take any meaningful action because Russia is a permanent veto-wielding member along with the United States, France, the United Kingdom and China.

Surveys in Russia have suggested widespread domestic backing for the army’s intervention in Ukraine – but mass conscription may be a risky move for Putin after past Kremlin promises it would not happen and a string of battlefield failures in Ukraine.

“Every normal person is (concerned),” said one man, identifying himself only as Sergey, disembarking in Belgrade after a flight from Moscow.

“It is OK to be afraid of the war.”

Anti-war protests in 38 Russian cities resulted in more than 1300 people being arrested on Wednesday, a monitoring group said.

Some of the detainees had been ordered to report to enlistment offices on Thursday, the first full day of conscription, independent news outlets said.

The Russian government said reports of a mass exodus were exaggerated.

Russian news agencies meanwhile reported on Thursday that 10,000 people had volunteered to fight even before their call-up papers had arrived, citing the Russian General Staff.

Putin’s defence minister has said the mobilisation is intended to enlist about 300,000 men.

Prices for air tickets from Moscow soared above $A7500 for one-way flights to the nearest foreign locations, with most sold out for coming days.

Traffic also surged at border crossings with Finland and Georgia.

On the ground, explosions shook southeastern Ukraine on the eve of the referendums planned there by pro-Russian separatists – votes that Ukraine and its allies have called a sham.

In the Russian-held southern city of Melitopol, in the Zaporizhzhia region, a blast hit a crowded market.

The city’s exiled mayor said it had killed three soldiers and was staged by occupying forces to accuse Ukraine of terrorism.

A member of the Russian-installed local administration accused Ukrainian special services of trying to cause chaos on the eve of voting.

In further violence, Russian-installed separatists said at least six civilians had been killed and six more injured in a missile strike on a market in central Donetsk.

Bodies lay on the street next to a wrecked bus surrounded by debris and twisted metal.

A firefighter hugged a sobbing woman whose daughter had been killed inside the vehicle.

A Reuters reporter saw five dead people in the separatist-controlled city, including a teenager, and several wounded.

-AAP