100,000 trapped in Mariupol as Ukraine pleads for help

Ukraine appealed for more Western military help ahead of a NATO summit, as it warned that almost 100,000 people are trapped by Russian bombardment and facing starvation in the ruins of the besieged port of Mariupol.

Tens of thousands of residents have already fled the southern city, bringing harrowing testimony of a "freezing hellscape riddled with dead bodies and destroyed buildings", according to Human Rights Watch.

As the UN demanded Russia end its "unwinnable" war, Ukrainian President Volodymyr Zelensky told the Japanese parliament that the UN Security Council was dysfunctional and in need of reform, after Russia wielded its veto to nix condemnation of its invasion.

Nearly a month on, peace talks have agreed on daily humanitarian corridors for refugees, and Ukraine says it is willing to countenance some Russian demands subject to a national referendum.

Ukraine's lead negotiator Mykhaylo Podolyak said the peace talks were encountering "significant difficulties", after Moscow accused the US of undermining the process.

Russia meanwhile refuses to rule out using nuclear weapons if it faces an "existential threat", Kremlin spokesman Dmitry Peskov told CNN.