"As we enter winter, any disruption of essential services, such as water and heating amid freezing temperatures could have grave consequences for millions of Ukrainians," Walker said in a statement.
"Today, the availability of clean water for 1.1 million people on both sides of the 459-kilometre 'contact line' dividing eastern Ukraine hangs in the balance. In the past six days, the Donetsk Filter Station (DFS) has been shelled for three consecutive nights. The Verkhniokalmiuska Filter Station (VFS) was hit more than 12 times in a single night. Water is continuing to flow due to the bravery of men and women, who risk their lives working for water company Voda Donbasa," he said.
Read alsoUN: Donbas rapidly becomes one of world's most mined areasIt is reported that such an escalation, so close to critical infrastructure, risks prompting a severe deterioration of the ongoing humanitarian crisis on Europe's doorstep. Without sustained essential water supply, heating systems will stop and health conditions will deteriorate. Children, the elderly, women and people with disabilities may flee their homes in search of heat and shelter.
The UN is also extremely concerned about the possible release of hazardous chlorine gas stored at both filter stations. If a single 900 kg gas container is hit, anyone within 200 meters might receive fatal dose of the poisonous gas.
"I remind all parties to the conflict of their obligation to respect civilian infrastructure and to protect civilians. Any intentional disruption of access to quality water supply or critical heating systems is a clear violation of International Humanitarian Law," Walker said.
"Today, we see the continued disproportionate impact of the crisis on ordinary Ukrainians. The conflict is denying the most vulnerable of their basic needs, and most importantly, safety and protection. In this protracted crisis, millions of families trapped in active hostilities and people displaced are losing hope for a future," he stressed.
"I call on all parties to uphold their responsibility to facilitate the access of humanitarian organizations to provide critical services for the most vulnerable Ukrainians affected and displaced by the conflict – now in its fourth year," he added.