The United Nations have called for a pause in the fighting in Aleppo to allow aid to get through to the millions left without power, water and medicine.

UN officials said there needed to be an urgent 48-hour break in hostilities to allow essential infrastructure to be repaired after airstrikes left more than two million residents without electricity or access to the public water network.

The organisation called for an end to the siege of the Syrian city and said that to deliberately deprive people of food and water constituted a "war crime".

"The U.N. is extremely concerned that the consequences will be dire for millions of civilians if the electricity and water networks are not immediately repaired," a joint statement from Yacoub El Hillo, U.N. Resident and Humanitarian Coordinator for Syria, and Kevin Kennedy, U.N. regional humanitarian coordinator for the Syria crisis, said.

Senior UN officials said that water available through wells and tanks in Aleppo is not nearly enough to sustain the needs of the population.

Unicef, warned of the risks of water shortages to children. In a statement, the organisation said: "These cuts are coming amid a heat wave, putting children at a grave risk of waterborne diseases."