Any peace deal must not 'sell out Ukraine': UK

Any peace deal between Kyiv and Moscow must not "sell Ukraine out" and should include provisions to automatically re-trigger sanctions if Russia acts aggressively, UK Foreign Secretary Liz Truss said Monday.

The minister said that lessons needed to be learned from the "uneasy settlement" reached following the unrest of 2014, when Russia annexed the Ukrainian peninsula of Crimea, which she said "failed to give Ukraine lasting security".

"Putin just came back for more. That is why we cannot allow him to win from this appalling aggression," she told parliament.

Instead, "we need to ensure that any future talks don't end up selling Ukraine out", she added.

Ukraine warned on Monday that the humanitarian crisis in the pulverized city of Mariupol was now "catastrophic", with thousands dead, as fighting surged around Kyiv ahead of new face-to-face peace talks with Russia in Turkey.

Truss said any long-term deal needed to include "a clear sanction snapback, which would be triggered automatically by any Russian aggression".

"We need to ensure that Putin can never act in this aggressive way again."

About 20,000 Ukrainians have been killed in Russia's month-old invasion and 10 million have fled their homes, according to Kyiv. Several cities are still coming under withering bombardment.

Ukraine President Volodymyr Zelensky said the first round of in-person talks since March 10 -- due to open in Istanbul on Tuesday after near-daily video contacts -- must bring peace "without delay".

Ukrainian "neutrality", and the future status of the Russian-speaking Donbas region in the east could be in the mix for the Istanbul meeting.