The United Nations says that four months into Russia's unprovoked invasion, the humanitarian situation in eastern Ukraine has reached an "extremely alarming" level and keeps worsening.

"Nearly four months since the start of the war, the humanitarian situation across Ukraine -- particularly in the eastern Donbas -- is extremely alarming and continues to deteriorate rapidly," the UN's humanitarian agency, OCHA, said in a statement.

OCHA said the situation was "particularly worrying in and around Syevyerodonetsk," the eastern Ukrainian city where intense fighting has raged for several weeks.

"In Syevyerodonetsk, as we have said, thousands of people -- including women, children, and the elderly -- are experiencing constant bombardments and clashes," the statement said.

"The parties to the conflict have not reached an agreement to either facilitate safe evacuations of civilians or enable access to aid workers to provide urgent assistance."

The UN said there was "diminishing access to clean water, food, sanitation, and electricity" in the city.

It said that as "active hostilities continued to escalate" in eastern Ukraine, the war was "taking an enormous toll on civilians, including aid workers."

"Despite enormous access challenges, the UN and humanitarian partners have reached over 8.8 million people across Ukraine since the war started," it added.

With reporting by AFP