KYIV — Ukrainian cities have begun imposing curbs on events linked to the country's Independence Day on Wednesday, amid fears of intensified Russian attacks.

The anniversary commemorates freedom from Soviet rule and also marks half a year since the Russian invasion.

The US State Department issued a security alert on Monday warning that Russia is stepping up efforts to launch strikes against civilian and government targets in Ukraine in the coming days.

American citizens still in Ukraine have been urged to leave the country immediately by the US Embassy in Kyiv.

The alert came after the US intelligence community declassified a finding on Monday, saying that Russia would increasingly target Ukrainian civilian infrastructure.

Kyiv has banned public events related to the occasion from Monday to Thursday. There are fears of renewed Russian rocket attacks on the capital, even though it lies far from the front lines and has only rarely been hit since Ukraine repelled a ground offensive in March.

In Kharkiv, the northeastern city that has come under frequent and deadly longer-range artillery and rocket fire, Mayor Ihor Terekhov announced an extension to an overnight curfew to run from 4 pm to 7 am effective from Tuesday to Thursday.

Curbs on events have also been imposed in the port of Mykolaiv, near Russian-held territory to the south. Regional governor Vitaliy Kim said there were plans for residents to be ordered to work from home on Tuesday and Wednesday, urging people not to gather in large groups.

Fears of attacks have mounted after Russia's Federal Security Service (FSB) accused Ukrainian agents on Monday of killing Daria Dugina, daughter of a Russian ultra-nationalist ideologue, in a car bomb attack near Moscow. Ukraine denies involvement.

Ukrainian President Volodymyr Zelensky has warned that Moscow could try "something particularly ugly" in the run-up to Independence Day.

The Ukrainian military said on Tuesday that Russia has carried out artillery and air strikes in the Zaporizhzhia region, where fighting near Europe's largest nuclear power plant has raised fears of a catastrophic nuclear incident.

Ukraine's General Staff said Russia fired artillery and conducted air strikes in several towns in the area, were Russian forces captured the nuclear power plant shortly after they invaded on February 24.

Artillery and rocket fire near the Zaporizhzhia nuclear reactor complex, on the south bank of the Dnipro River, has led to calls for the area to be demilitarized.

People living nearby said they were worried about the potentially disastrous consequences if one of the plant's six reactors was hit by shells.

Russia and Ukraine repeatedly accuse each other of firing at the Zaporizhzhia facility, which lies close to the town of Enerhodar that pro-Moscow forces now control.

"It's like sitting on a powder keg," town resident Alexander Lifirenko said on Monday.

The head of the city's civil-military administration said local authorities had a plan to cope with an accident and noted some shells were falling close to the plant's reactors.

"In case a reactor is hit, it will be not a local, but a global catastrophe," said Alexander Volga. — Agencies