Russia might take the provocative step of putting Ukrainian soldiers on trial as Kyiv marks 31 years of independence for the war-ravaged country next week, Ukraine's President Volodymyr Zelensky warned Sunday.

Zelensky cited media reports that Russia was preparing to put Ukrainian fighters captured during the siege of Mariupol on a public trial to coincide with the independence anniversary Wednesday.

Ukraine's Independence Day, August 24, will also mark six months since Russia invaded the former Soviet republic, in a devastating war that has cost thousands of lives.

"If this despicable court takes place, if our people are brought into these settings in violation of all agreements, all international rules, there will be abuse," Zelensky warned in an evening address.

"This will be the line beyond which no negotiations are possible."

The capital Kyiv has already announced a ban on public gatherings. Kharkiv too, declared a curfew around the holiday.

Zelensky was returning to a subject he had already raised in the previous night's remarks.

"Russia could try to do something particularly disgusting, particularly cruel," he warned late Saturday.

"One of the key objectives of the enemy is to humiliate us," and "to sow despondency, fear and conflict".

But he added: "We have to be strong enough to resist all provocation" and "make the occupiers pay for their terror".

'Be vigilant'

A presidential adviser, Mykhaylo Podolyak, said Russia could intensify its bombing campaign.

"Russia is an archaic state that links its actions to certain dates, it's an obsession of sorts," the Interfax-Ukraine news agency quoted him as saying.

"They hate us and will try to increase... the number of bombings of our cities including Kyiv with cruise missiles," Podolyak added.

Kyiv authorities on Sunday banned public gatherings from August 22 to August 25.

In the northeastern city of Kharkiv, the regional governor announced a curfew from the evening of August 23 to the morning of August 25.

"We will not allow any provocation by the enemy. Be as vigilant as possible during our independence holiday," Oleg Synegubov wrote on Telegram.

Kharkiv has been under regular Russian bombardment for weeks and on Sunday emergency services said a woman was killed and two other civilians were wounded in overnight strikes.

Four civilians were reported killed by Russian fire in Donetsk, said the region's pro-Kyiv governor, Pavlo Kyrylenko.

Five cruise missiles were fired at Odessa from the Black Sea, a regional administration spokesman said. Two were shot down by air defences and three hit a silo without causing injury.