By Stephanie Nebehay

GENEVA (Reuters) - The United Nations called on Tuesday for an urgent ceasefire in the divided Syrian city of Aleppo, where it said two million people lack access to clean water, putting people, especially children, at risk of disease.

Access is needed to deliver food and medical supplies and for technicians to repair electricity networks that drive water pumping stations, which were heavily damaged in attacks last week.

"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," it said in a statement, adding that, at minimum, a weekly 48-hour pause was needed.

It was signed by U.N. Resident and Humanitarian Coordinator for Syria Yacoub El Hillo, and Kevin Kennedy, U.N. regional humanitarian coordinator.

An estimated 250,000-275,000 people are trapped in eastern Aleppo following the closure of Castello road last month, the last remaining access route to the opposition-held part of the city, the statement said.

Insurgents effectively broke a month-long government siege of eastern Aleppo on Saturday. Their advance severed the primary government supply corridor running into the city from the south and raised the prospect that government-held western Aleppo might in turn become besieged by the insurgents.

This brought the total number of civilians in the city under "de facto fear of besiegement to over two million," the U.N. said.

Later, pro-Damascus media reported that Syrian government forces and their allies had captured territory from rebels in southwestern Aleppo, closing off a rebel route into the east.

U.N. Special Envoy for Syria Staffan de Mistura and U.N. aid chief Stephen O'Brien are to brief the Security Council on the deteriorating situation in Aleppo on Tuesday.

CHILDREN AT RISK

Young children are especially vulnerable to diarrhea and other water-borne diseases from drinking dirty water, the U.N. Children's Fund (UNICEF) said.

"In the eastern parts of Aleppo up to 300,000 people - over a third of them are children - are relying on water from wells which are potentially contaminated by fecal matter and unsafe to drink," UNICEF spokesman Christophe Boulierac told a briefing.

UNICEF and other aid agencies are bringing emergency drinking water by truck to an estimated 325,000 people in western Aleppo per day, he said.

"The water and electricity situation is alarming," said Krista Armstrong of the International Committee of the Red Cross (ICRC).

The ICRC and Syrian Arab Red Crescent have distributed millions of litres of drinking water to 70,000 residents and displaced people in recent days and to some of the city's main hospitals, she said.

But Aleppo's sick and wounded lack access to medical care following at least 10 attacks on health facilities in July, the World Health Organization said.

"Only 35 medical doctors remain in the city, and their capacity to cope with the excess demand has been overwhelmed," WHO spokesman Tarik Jasarevic said.

(additional reporting by Brenna Hughes Neghaiwi; Editing by Louise Ireland and Robin Pomeroy)