The UN's crisis coordinator in Ukraine Sunday called for an immediate stop to fighting in besieged Mariupol to allow trapped civilians to leave the city safely.

"The lives of tens of thousands, including women, children, and older people, are at stake in Mariupol," said Amin Awad.

"We need a pause in fighting right now to save lives. The longer we wait the more lives will be at risk. They must be allowed to safely evacuate now, today. Tomorrow could be too late."

Amin reiterated UN Secretary-General António Guterres' appeal for a halt to fighting to allow life-saving supplies into Mariupol and other areas under attack and enable the departure of those wanting to leave amid reports of the situation in Mariupol deteriorating dramatically.

"At a time of a rare calendar alignment of the religious holidays of Orthodox Easter, Passover and Ramadan, it is the time to focus on our common humanity, setting divisions aside," he said.

According to the UN agencies, the humanitarian situation in Ukraine continues to worsen, as a result of ongoing hostilities chiefly taking place in eastern and southern regions.

Over a quarter of Ukraine's population is now displaced internally or in neighbouring countries.

The latest assessment of the International Organization for Migration found that around 60 percent of those displaced are women, and more than half of those displaced expressed their need for food.

The World Food Programme said over one-quarter of internally displaced families with children under the age of five had reported difficulties in feeding their children since the start of the conflict.

As hostilities intensify, the basic needs of people stranded in hard-to-reach areas increase, while also hampering efforts to establish humanitarian corridors to both evacuate civilians and deliver life-saving assistance.

The most urgent and critical needs are in these locations with high levels of insecurity, disrupted food supply and retail capacities, and limited or sporadic humanitarian access, such as Mariupol.