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Fears are growing about the trapped residents as both sides bolster their forces for what has been predicted to be the "mother of all battles" for Aleppo.

Once Syria's largest city and industrial hub, control of Aleppo has been a focal point for rival factions in the country's five-year civil war.

Rebel ranks and President Bashar al-Assad's regime have sent hundreds of reinforcements to Aleppo in anticipation of the fighting.

It comes after opposition forces broke a government siege at the weekend and vowed to capture the entire city.

Hundreds of thousands of civilians remain inside the city and UN officials have sounded the alarm over trapped residents.

The UN's top humanitarian official in Syria, Yacoub El Hillo, and regional coordinator Kevin Kennedy said in a statement late on Monday that medical and food stocks "are running dangerously low" in Aleppo.

They appealed for a full-fledged ceasefire or weekly 48-hour "humanitarian pauses" to reach those in need.

Blackout in Aleppo: Attacks in July struck an electricity transmission station [AFP]

The UN says two million people in the city are at risk, including up to 275,000 people in opposition-held east Aleppo. Other estimates put the total number of civilians in the city at about 1.5 million, with 250,000 in the eastern districts.

Four days without running water

The UN has warned that residents inside Aleppo have gone without running water for the past four days.

Temperatures in Aleppo were pushing a sweltering 40 degrees Celsius [104 degrees Fahrenheit] on Tuesday, according to BBC Weather.

The UN's children agency said attacks on 31 July struck the electricity transmission station, which powered water pumping stations to eastern and western parts of the city.

Authorities had urgently restored an alternative power line on 4 August, only to see it damaged less than 24 hours later, with intense fighting hampering repair efforts.

"As a result, the whole city has been without running water for four days," the UNICEF statement said.

Aid agencies are scaling up emergency response to bring safe drinking water to civilians, but warn that people will soon be forced to resort to unsafe water sources

The agency said it and its partners were scaling up their emergency response to bring safe drinking water to civilians. It also warned that people would soon be forced to resort to drinking polluted, unclean water if the pumping systems were not quickly repaired.

Children at risk

UNICEF said children and families were facing "a catastrophic situation", amid cuts to water and electricity supplies.

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"These cuts are coming amid a heatwave, putting children at a grave risk of waterborne diseases," Hanaa Singer, UNICEF's representative in Syria, said in a statement.

"Getting clean water running again cannot wait for the fighting to stop. Children's lives are in serious danger."

The recent flare-up in fighting began in late June as government forces closed in on the Castello Road, the last route into rebel-held parts of the city.

The offensive has left residents across the city reeling from skyrocketing prices and food shortages, afraid of further violence.

Both the regime and rebel sides used newly acquired territory to bring food and other supplies into neighbourhoods they control, but the roads are still not safe for civilians to use.

Emboldened by their recent win, the rebel alliance on Sunday announced an ambitious bid to capture all of Aleppo city, which if successful would mark the biggest opposition victory yet in Syria's conflict.

'Mother of all battles'

"The battle for Aleppo is arguably the most emotive and strategic of any across Syria," Charles Lister, senior fellow at the Middle East Institute, wrote in an titled "Aleppo - The Mother of All Battles."

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Sporadic clashes hit the edges of the city on Tuesday, but there were no signs yet of either side launching a large-scale offensive.

Yasser Abdulrahim, a rebel commander who leads a joint operations room for Aleppan fighters, said preparations were still underway.

"The big battle has not started yet," Abdulrahim told AFP. "We are waiting for more reinforcements before it begins, and we are trying to find the weakest points in our enemy's lines."

He said clashes were taking place in the southern suburbs of Aleppo, in the key district of al-Ramousa and a collection of military academies.

"Most of the clashes in Ramousa are taking place against Hizballah and Iranian fighters," Abdulrahim said, referring to Assad's backers in the Lebanese Shia movement and Tehran.

A military source in Damascus told AFP that regime forces had moved within firing range of the academy complex, where rebel forces broke though government lines on Saturday.

The Syrian Observatory for Human Rights, a UK-based monitoring group, confirmed clashes were taking place in Ramussa and said air raids on a rebel-held district of Aleppo killed three civilians on Tuesday morning.

Agencies contributed to this report.