The Ukrainian port city of Odesa on Saturday restored power to critical infrastructure after a fire broke out at an overloaded substation, leaving nearly 500,000 people without electricity, a top official said.

The blaze, which erupted earlier in the day, is a new blow to the country's ailing energy grid that has been hammered by Russian strikes for months.

Officials said repairs could take weeks. The government said it would appeal to Turkey for help.

"Power to all critical infrastructure has been restored. The city will therefore have water and heat," Ukraine Energy Minister German Galushchenko wrote on Facebook late in the day.

"About a third of the city's consumers now have lighting," he said, without giving precise details, adding that 31 high-power generators were on their way to the city.

The CEO of the state grid operator, Volodymyr Kudrytskyi, earlier said critical equipment that had already been damaged several times by Russian missile strikes burst into flames when it could no longer "withstand the load".

He told a briefing that any further Russian missile or drone attacks could make matters even worse.

Since October, Moscow has waged a campaign of massive missile attacks on energy infrastructure. Moscow says the strikes aim to reduce Ukraine's ability to fight; Kyiv says they have no military purpose and are intended to hurt civilians.

"The situation is difficult, the scale of the accident is significant, it is impossible to quickly restore power supply," Prime Minister Denys Shmyhal wrote on Telegram.

Odesa regional governor Maksym Marchenko said practically all of the city had lost power after the incident, and that as of Saturday afternoon about 500,000 people faced outages.

That represents about half of Odesa's pre-war population of one million, when it was Ukraine's third largest city.

"Today's power supply (availability) allows to supply the city and the district about 40 or 45%, but if we factor in critical infrastructure, then of course very little is left for ordinary citizens," Kudrytskyi said.

The temperature in Odesa stood at two degrees Celsius on Saturday and is due to dip below freezing for much of next week.

Shmyhal said he had ordered Ukraine's foreign ministry to appeal to Turkey to send powerships – vessels that carry power plants – to come to the city's aid.