President Vladimir Putin introduced martial law on Wednesday in four Ukrainian regions he says Russia has annexed as some residents of the occupied city of Kherson left by boat following warnings of a looming assault.

The images of people fleeing Kherson were broadcast by Russian state TV which portrayed the exodus – from the right to left bank of the River Dnipro – as an attempt to clear the city of civilians before it became a combat zone.

Kirill Stremousov, deputy head of the local Russia-backed administration, made a video appeal after Russian forces in the area were driven back by 20-30 km in the last few weeks. They risk being pinned against the western bank of the 2,200-km-long Dnipro river that bisects Ukraine.

In a move which looked designed to help Russia firm its grip on the Ukrainian regions it partly occupies – including Kherson – Putin told his Security Council he was introducing martial law in them.

Beyond much tighter security measures on the ground, it was unclear what the immediate impact of that would be.

Kyiv, which does not recognise Moscow's self-styled annexations of the four regions, derided the move.

"'Martial law' implementation on the occupied territories by Russia should be considered only as a pseudo-legalisation of (the) looting of Ukrainians’ property," tweeted Mykhailo Podolyak, a Ukrainian presidential adviser.

"This does not change anything for Ukraine: we continue the liberation and deoccupation of our territories." (Reuters)