Russia's Putin declares martial law in seized Ukrainian regions

President Vladimir Putin introduced martial law on Wednesday in four Ukrainian regions he says are part of Russia as some residents of Kherson left by boat after Moscow warned of a looming assault.

The images of people fleeing Kherson were broadcast by Russian state TV which portrayed the exodus - from the right bank of the Rover Dnipro to its left bank - 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 four Ukrainian regions it partly occupies and seeks to fully control - including the Kherson region - 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.

Putin also issued a decree restricting movement in and out of eight regions adjoining Ukraine.

Andriy Yermak, the head of the Ukrainian president's office, accused Russia of laying on a propaganda show in Kherson.

"The Russians are trying to scare the people of Kherson with fake newsletters about the shelling of the city by our army, and also arrange a propaganda show with evacuation," Yermak wrote on the Telegram messaging app.

Eight months after being invaded, Ukraine is prosecuting major counter-offensives in the east and south to try to take as much territory as it can before winter after routing Russian forces in some areas.

Kherson is the biggest population centre Moscow has seized and held since it began its "special military operation" in Ukraine on February 24. The city is on territory which Putin says is now formally incorporated into Russia, a move Ukraine and the West do not recognise.

The conflict has killed thousands, displaced millions, pulverised Ukrainian cities, shaken the global economy and revived Cold War-era geopolitical fissures.

Ukrainian cities have also been struck in recent days by drones and missiles, and Vitali Klitschko, Kyiv's mayor, said the capital's air defences were in action once again on Wednesday.

In Kherson, Stremousov said the city and especially its right bank could be shelled by Ukrainian forces, adding that residents who left would be given accommodation inside Russia.

"I ask you to take my words seriously and to interpret them as a call to evacuate as fast as you possibly can," he said.

"We do not plan to surrender the city, we will stand until the last moment."