Ukraine is facing its "toughest winter" since Russia's full-scale invasion in 2022, a top United Nations official told the BBC.

Alexander De Croo, head of the UN development agency, said there was a risk of "humanitarian catastrophe" if Russian attacks cut off access to power and water.

Speaking in Ukraine, De Croo – the UN's third most senior official – accused Russia of systematically targeting the country's energy infrastructure. He said there was now a "race against the clock" to repair the power network before winter sets in and temperatures plummet.

Russia and Ukraine have been intensifying aerial attacks in recent months, and both deny deliberately targeting civilian infrastructure.

Russia has started to use faster, jet-powered drones that are harder to intercept in recent around-the-clock attacks on Ukraine - and in particular Kyiv. Many of the strikes have targeted Ukraine's infrastructure, including businesses, warehouses, factories, petrol stations, as well as energy infrastructure.

Ukraine continues to launch drone strikes deep into Russian territory, largely against gas, oil and business distribution facilities. Both countries deny targeting civilians.

Deadly attacks have resumed after US President Donald Trump sent envoys for talks in Moscow with Russian President Vladimir Putin and in Kyiv with Ukrainian President Volodymyr Zelensky last weekend.

Five people were killed and 67 others were wounded in a daytime Russian drone strike on a shopping mall in Pavlohrad, a city in Dnipropetrovsk region, on Thursday, Ukrainian officials say. Earlier in the week two other shopping malls were targeted by Russian air strikes, killing two people and injuring 28 people in the attacks on the cities of Zaporizhzhia and Sumy.

Russia reported a Ukrainian seaborne drone attack on the resort city of Sochi overnight on Wednesday, in which 28 people were wounded, as well as three more deaths blamed on Ukrainian attacks.

Two people were also killed earlier this week when Russian drones hit a border crossing between Ukraine and Moldova, which has increasingly found itself caught up in the conflict.

A full-scale war has been raging between Russia and Ukraine since Putin launched an invasion more than four and a half years ago.

De Croo, a former prime minister of Belgium, told the BBC: "This winter is going to be the toughest one since the war".

He said Ukraine started "from a very weak base going into this winter". He added the UN were "very, very worried... about what this winter could become."

The concerns are driven by what De Croo said was Russia's "very systematic" targeting of civilian and energy infrastructure.

"This is petrol stations, it's shopping malls, it's business centres, but it's especially energy infrastructure, energy transmission lines, and so on. There is absolutely no justification for that," he said.

This could lead a "worst-case scenario" where people struggled to access energy to run heating, and water and sewage systems stopped working.

"With the temperatures that you would get here that would lead to circumstances which would be really close to humanitarian catastrophe," De Croo said.

Winter temperatures in Ukraine can drop to -20C.

If power supplies and critical infrastructure were severely damaged it could lead to "quite some human suffering", De Croo said.

Under efforts to prepare Ukraine for winter the UN Development Programme is helping to rebuild parts of its energy system that were damaged last winter. It is also establishing more sustainable and decentralised energy plants, which are less vulnerable to attack.

De Croo said completing the difficult and dangerous work was "almost an impossible task".

"They are reconstructing these sites under constant, constant air alarms. And they know that they are working at a site which is a target," he said. "So you try to reconstruct, but part of your day you can't work because it's just too dangerous."