United Nations Secretary-General Antonio Guterres made a last-minute plea on Wednesday to Russian President Vladimir Putin to stop the war 'in the name of humanity', after the Russian leader announced a military operation in eastern Ukraine.

"President Putin, in the name of humanity, bring your troops back to Russia," Guterres said, speaking after an emergency meeting of the Security Council on Ukraine.

The consequences of a war would be devastating for Ukraine and far-reaching for the global economy, he added.

Speaking after Guterres, UN Under-Secretary for Political Affairs, Rosemary DiCarlo, said Wednesday there were reports of "large-scale military buildup and military columns moving towards Ukraine" and that Russia had "shut airspace" to civilian aircraft along the border between the two countries.

The UN could not verify the reports, she added.

As world leaders one after another urged Russia to stand down, Putin made a surprise speech on Russian TV, announcing he had "made the decision of a military operation."