"The targeting of hospitals and clinics continues unabated, seriously jeopardizing the health and welfare of all citizens of Aleppo," said Yacoub al-Hillo, UN resident and humanitarian coordinator for Syria, and Kevin Kennedy, regional humanitarian coordinator for the Syria Crisis.
Attacks on civilian infrastructure this week severely damaged the city's electric and water infrastructure, leaving more than two million residents of Aleppo without electricity or access to the public water network, they said.
"The UN is extremely concerned that the consequences will be dire for millions of civilians if the electricity and water networks are not immediately repaired," Hillo and Kennedy warned.
They urged the immediate lifting of all sieges and a total ceasefire or weekly 48-hour humanitarian pauses to reach millions of people in need throughout Aleppo and resupply food and medicines "which are running dangerously low".
UN children's charity Unicef said the whole city had been without running water for four days.
"Children and families in Aleppo are facing a catastrophic situation," said Hanaa Singer, Unicef's representative in Syria.
"Getting clean water running again cannot wait for the fighting to stop. Children's lives are in serious danger."
Fighting between government forces and rebels has intensified in recent weeks and on Sunday rebels cut off the government's key access route into western Aleppo, prompting an intense campaign of air strikes by pro-government forces.
In July there were at least 10 confirmed attacks on health facilities in Aleppo and only 35 medical doctors remain in the city, according to the UN World Health Organisation.
At least 250,000 people are believed to have been trapped in rebel-held areas of eastern Aleppo since a key access route into the area was closed by government forces in early July.
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