Damaged pipelines in war-torn eastern Ukraine are threatening to cut off water, heating and electrical supplies at a time when temperatures are plunging, putting some 700,000 children at risk, UNICEF warned Friday.

The UN children's agency pointed out that central heating systems and much of the electrical supply in the Donetsk and Lugansk regions, under pro-Russian separatist control, rely on water, but that the water flow is being hampered by damaged pipelines and infrastructure, as well as debris from destroyed bridges.

"At the moment, about 700,000 children are at risk, because of the potential failure of the central heating systems in that area during the freezing winter months," said William Fellows, head of UNICEF's water and sanitation division in Ukraine.

Those children are among nearly five million people living in the Donetsk and Lugansk region who are "totally dependent on two centralised water systems, both of which actually cross the front line of the conflict, and both systems have been damaged heavily during the conflict," Fellows told reporters in Geneva via telephone link from Kiev.

Some parts of the network have been repaired, but other areas remain heavily mined and unaccessible, he said.

Fellows stressed the urgency of repairing the water network, pointing out that temperatures in the region routinely plunge to minus 20 degrees Celsius (-4 degrees Fahrenheit), and even as low as minus 30 (-22 degrees Fahrenheit).

"Unless these repairs are carried out before winter hits hard, we run the very real risk of the water supplies freezing and consequently the failure of the heating systems," he said.

But gaining access to all the areas where the network has been damaged has been difficult.

In a statement, UNICEF on Friday called for "safe humanitarian access to non-government-controlled areas in eastern Ukraine to provide life-saving supplies to children, remove UXO (unexploded ordnance) and urgently repair the infrastructure to keep the water supply, sewage and heating systems functioning."

The Organization for Security and Co-operation in Europe (OSCE) has also warned that eastern Ukraine could face severe shortages of water, and thus heat, this winter due to the destroyed pipelines.