The Russian-installed chief of Kherson said about 50,000-60,000 people would be moved out in the next six days.

Donetsk, Luhansk and Zaporizhzhia were also under martial law.

“The Ukrainian side is building up forces for a large-scale offensive,” Vladimir Saldo, the official, told state TV. “Where the military operates, there is no place for civilians.”

It was unclear what the immediate impact of Putin’s declaration of martial law would be, beyond much tighter security measures in Kherson and the other three regions.

Kyiv, which along with the West does not recognise Moscow’s self-styled annexations, 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.”

Staff at Kherson’s Russian-backed administration were also being relocated to the left bank of the Dnipro, he said, although he said Russia had the resources to hold the city and even counter-attack if necessary.

Eight months after being invaded, Ukraine is pressing 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.

Russian forces near Kherson have been driven back by 20-30 km in the last few weeks and risk being pinned against the western bank of the 2200km Dnipro, which bisects Ukraine.

In televised remarks to his Security Council, Putin boosted the powers of Russia’s regional governors and ordered the creation of a coordinating council under Prime Minister Mikhail Mishustin to support his “special military operation”.

He said the “entire system of state administration”, not only the specialised security agencies, must be geared to back up the Ukraine effort.

The sides exchanged intermittent artillery fire on a section of the Kherson front in the Mykolaiv region on Wednesday, the impacts marked by towers of smoke.

Several Ukrainian soldiers said they were aware of the martial law declaration but were not worried, although they warned a visiting Reuters reporter of the danger of Russian drones.

“For sure he’s (Putin) up to no good. We understand that,” said Yaroslav, who declined to give his last name. “But whatever they are doing, we will screw them anyway.”

Oleh, who also withheld his last name, said Russia in the past had warned about what it claimed would be escalatory Ukrainian actions only to carry them out itself.

“We are just concerned about our people in the Kherson region,” he said.

Moscow denies deliberately targeting civilians, though the conflict has killed thousands, displaced millions, pulverised Ukrainian cities, shaken the global economy and reopened Cold War-era geopolitical fissures.

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

Ukrainian President Volodymyr Zelenskiy, who has said a third of his country’s power stations have been hit by Russian strikes, discussed security at power supply facilities with senior officials.

“We are working to create mobile power points for the critical infrastructure of cities, towns and villages,” Zelenskiy wrote on the Telegram messaging app.

“We are preparing for various scenarios of possible consequences. Ukraine will defend itself. No matter what the enemy plans and does.”

-AAP