The United Nations on Thursday condemned what it called a "particularly troubling" wave of attacks in Ukraine against first responders who rush to the scene of air strikes, only to be attacked themselves.

In these second wave strikes, those wounded in the first strike are killed and first responders are killed or injured, UN official Edem Wosornu told the Security Council, speaking on behalf of humanitarian chief Martin Griffiths.

"This compounds the suffering of the wounded and incapacitates first responders," she said.

"Attacks directed against the wounded and those helping them are prohibited by international humanitarian law. They are cruel, unconscionable, and must stop."

In recent weeks, Ukraine has repeatedly accused Russia of carrying out such attacks -- bombing a location, and then hitting it again once emergency personnel are on site.

"Ukraine is currently enduring some of the worst attacks since the start of this war," Wosornu said.

"Not a day passes without air strikes shattering the lives of yet more families across the country."

For Miroslaw Jenca, UN assistant secretary-general for Europe, "the current trajectory of escalation of this war is a direct threat to regional stability and international security."

"Most of all, it is an existential threat to the people of Ukraine," Jenca said, making specific reference to attacks on Ukraine's energy installations.