United Nations Secretary-General Antonio Guterres warned the body that "our world is facing a moment of peril" over Ukraine's crisis with Russia.

"If the conflict in Ukraine expands, the world could see a scale and severity of need unseen for many years," he said.

Guterres called for "all sides to allow safe and unimpeded access by humanitarian agencies, including in non-government controlled areas of eastern Ukraine."

He called Russia's recognition of the breakaway regions of Donetsk and Luhansk as independent states as "violations of the territorial integrity and sovereignty of Ukraine."

"It is time for restraint, reason and de-escalation," Guterres said, stressing there was no room for actions or statements that would "take this dangerous situation over the abyss."

Ukrainian Foreign Minister Dmytro Kuleba warned diplomats that "the beginning of a large-scale war in Ukraine will be the end of the world order as we know it."

He pleaded with the United Nations to hold Moscow accountable for what he called its attack on "the core principles of international law."

"If Russia does not get a severe, swift and decisive response now, this will mean a total bankruptcy of the international security system and international institutions, which are tasked with maintaining the global security order.

"This is a grim scenario, which will throw us back to the darkest times of the 20th century," he said.

Kuleba called Russian claims that it is acting to prevent planned military operations by Kyiv in the Donbas area, which incorporates Donetsk and Luhansk, as "absurd."

"Ukraine has never threatened or attacked anyone," he said. "Ukraine has never planned and does not plan any such action."

He called on Russia to withdraw troops from Ukrainian soil and pursue diplomacy.

"Russia must stop destabilizing the international security situation. We Ukrainians want peace and we want to resolve all issues through diplomacy," Kuleba said.