BEIRUT -- The battle for Syria's contested city of Aleppo crippled its infrastructure and left 2 million residents without running water, the U.N. children's agency said on Tuesday.

The dire warning came as Syrian activists said government warplanes were bombarding opposition-held parts of the city and the United Nations called for a "humanitarian pause" in the fighting so that electricity and water networks could be repaired. An intensification in fighting had damaged the electrical lines which pump water in the city, CBS News' Pamela Falk reports, and Aleppo had been without running water for 4 days, according to the regional UNICEF office.

Rival parties, however, were seen bringing in reinforcements to Aleppo ahead of an expected new round of fighting after opposition groups breached a government-imposed siege and cut a key government route to the city over the weekend.

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The battle for Aleppo, the country's largest city and former commercial heart, is pivotal for the Syrian civil war. It is not clear whether the rebels will be able to keep their new gains, but the breach causes a dent in the Syrian government's new confidence and territorial expansion, bolstered by Russian air support.

On Tuesday, two U.N. aid officials, Yacoub El Hillo and Kevin Kennedy, said in a statement that the water and electricity infrastructure had been damaged in the fighting. They said that the total number of civilians living in "de facto fear of besiegement" is now over two million.

"At a minimum, the UN requires a full-fledged ceasefire or weekly 48-hour humanitarian pauses to reach the millions people in need throughout Aleppo and replenish the food and medicine stocks, which are running dangerously low," the two said in a statement from Damascus.

With fighting disabling the power to the city's water pumping facilities, Aleppo residents have had access to running water for less than 24 hours since the end of last month.

UNICEF's Syria representative, Hannah Singer, said "children and families in Aleppo are facing a catastrophic situation. These cuts are coming amid a heat wave, putting children at a grave risk of waterborne diseases. Getting clean water running again cannot wait for the fighting to stop. Children's lives are in serious danger."

The World Health Organization said more than 15 Aleppo doctors who were outside the city when the government laid siege to the eastern part of it in mid-July and were now unable to return. The organization said it can confirm at least 10 attacks on medical facilities in eastern Aleppo in July alone.