An urgent situation is developing at the Azot chemical plant in the city, where it is believed hundreds of people are seeking refuge in makeshift bunkers beneath the plant.
A spokesperson for UN Humanitarian Affairs office, Saviano Abreu, told the BBC that the embattled city is running out of basic services.
The UN’s humanitarian branch said it was starting to making preparations for aid to be delivered to those trapped in the city.
However, with intense fighting showing no signs of easing up, the UN said it does not currently have access or assurances to safely get to civilians.
“The lack of water and sanitation is a big worry. It’s a huge concern for us because people cannot survive for long without water,” Mr Abreu told BBC News.
He also said food and health services are at risk of running out.
Russia said it would establish a humanitarian corridor on Wednesday in a bid to evacuate civilians trapped in the plant, however there has been no confirmation this has gone ahead.
The Kremlin said that civilians would be taken to the separatist region of Luhansk, in the Donbas.
The governor of the Luhansk region, Serhiy Haidai, said that around 500 civilians remain on the grounds of the Azot plant, 40 of which are children.
Severodonetsk, a key manufacturing centre and has been under intense bombardment for the past few weeks as the fight for control of the Donbas intensifies.