Zelenskiy began his much-anticipated speech by video to the UN General Assembly on Wednesday hours after Russian President Vladimir Putin announced a partial mobilisation of reservists to further his war against Ukraine.

While Zelenskiy had yet to address the development, he said his forces "can return the Ukrainian flag to our entire territory. We can do it with the force of arms. But we need time".

He also demanded punishment for Russia, including barring Russia from voting in international bodies and exercising its Security Council veto.

"So long as the aggressor is a party to decision-making in the international organisations, he must be isolated from them," Zelenskiy said.

The war has dominated the global gathering, which comes nearly seven months after Russia launched what has become the largest military conflict in Europe since World War II.

In a reflection of the circumstances, Zelenskiy wasn't at the rostrum where other presidents, prime ministers and monarchs speak at international diplomacy's most prominent annual gathering. Instead, he got an exception to speak via video.

As a permanent member of its most powerful entity, the Security Council, Russia was able to veto a demand to stop its attack on Ukraine days after it began.

Putin's decree on Wednesday about the partial mobilisation was sparse on details. Officials said as many as 300,000 reservists could be tapped.

Zelenskiy's speech was one of the most keenly anticipated at a gathering that has dwelled this year on the war in his country. But it wasn't the first time the first-term president has found himself in the spotlight at the assembly's annual meeting of presidents, premiers, monarchs and foreign ministers.

At last year's General Assembly meeting, Zelenskiy memorably compared the UN to "a retired superhero who's long forgotten how great they once were" as he repeated appeals for action to confront Russia over its 2014 annexation of Ukraine's Crimean peninsula and its support for the separatists.

Australian Associated Press