Ukraine's President Volodymyr Zelensky has called for punishing Russia for invading his country and making it compensate for the damages it has caused.

Speaking to the UN General Assembly's high-level session through a video link on Wednesday, Zelensky said a special international tribunal should be set up for its aggression so that it "will become (a) signal to all 'would-be' aggressors, that they must value peace or be brought to (take) responsibility by the world".

In the Assembly chamber full of formally dressed leaders and diplomats, Zelensky was projected on two giant screens dressed in an olive green T-shirt and spoke in Englsih while sitting informally at a desk.

All participants are required to speak in person at the meeting, but he was given special permission through an Assembly resolution backed by India last week to speak remotely despite Russia's opposition.

Zelensky accused Moscow of using oil and gas as "energy weapons" that are causing a cost-of-living crisis in several countries.

Prices of Russia's energy exports should be capped to end its blackmail, he said.

Zelensky's speech came after setbacks to Russia which lost about 3,700 square miles of seized territory to a counteroffensive and its President Vladimir Putin called up about 300,00 reservists to bolster its armed forces.

Biden calls for global resolve

US President Joe Biden took his attack on Russia to the UN General Assembly's high-level meeting accusing it of making nuclear threats while waging a "brutal war" against Ukraine.

Focusing on Russia's President Vladimir Putin, he said on Wednesday that it's "a brutal, needless war - a war chosen by one man, to be very blunt".

"A permanent member of the United Nations Security Council invaded its neighbour, attempted to erase a sovereign state from the map," he said.

"Just today, President Putin has made overt nuclear threats against Europe and a reckless disregard for the responsibilities of the non-proliferation regime," he said.

To ensure that no country can "seize a nation's territory by force", Biden said "each of us in this body who is determined to uphold the principles and beliefs we pledge to defend as members of the United Nations a" must be clear, firm, and unwavering in our resolve".

"We will stand in solidarity with Ukraine. We will stand in solidarity against Russia's aggression...," he added.

Biden also returned to this favourite theme of "democracy versus autocracy".

"The United States - and I, as President - champion a vision for our world that is grounded in the values of democracy," he said.