Russia has probably renewed its efforts to advance south of Ukraine's eastern city of Izium in the last 48 hours, Britain's defence ministry said on Saturday.

Its goal is to penetrate deeper into the Donetsk region and envelope the pocket around the embattled city of Sievierodonetsk from the north, it said on Twitter.

Zelensky pleads for arms as Russian forces lay siege to Severodonetsk

If trapped Ukrainian civilians do not take up an offer of leaving via a corridor, Russia is likely to claim justification in making less of a distinction between them and any Ukrainian military targets in the area, the ministry added.