Beirut - The United Nations has called for urgent aid access to Aleppo, warning that civilians are at grave risk from severe water shortages and disease after fighting intensified for Syria's second city.Fears are growing for trapped civilians ahead of what is expected to be an all-out battle for control of Aleppo, a focal point of Syria's five-year civil war.Rebels and regime fighters have sent hundreds of reinforcements to Aleppo in anticipation of the fighting after opposition forces broke a government siege at the weekend and vowed to capture the entire city.Hundreds of thousands of civilians remain inside Aleppo, once Syria's main economic hub and UN officials sounded the alarm.UN agencies said on Tuesday that up to two million people had gone without running water for four days, raising the risks of disease in a city already devastated by years of fighting.Unicef said children and families were facing "a catastrophic situation" after fighting damaged electricity networks needed to pump water."These cuts are coming amid a heatwave, putting children at a grave risk of waterborne diseases," said Hanaa Singer, its representative in Syria."Getting clean water running again cannot wait for the fighting to stop. Children's lives are in serious danger."The UN's top humanitarian official in Syria, Yacoub El Hillo, and regional coordinator Kevin Kennedy said medical and food stocks "are running dangerously low"."At a minimum, the UN requires a full-fledged ceasefire or weekly 48-hour humanitarian pauses to reach the millions of people in need throughout Aleppo and replenish the food and medicine stocks," they said.At a meeting of the UN Security Council, the United States and France urged for aid to reach Aleppo before a new diplomatic push to end the conflict can go ahead - though Russia said there should be no pre-conditions for the talks in Geneva.'Cut off'Aleppo has been divided between a rebel-held east and regime-controlled west since fighting erupted in the city in mid-2012.The UN says two million people in the city are at risk, including up to 275 000 people in east Aleppo. Other estimates put the number of civilians in the city at about 1.5 million, with 250,000 in the eastern districts.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 road was severed in mid-July, beginning a roughly three-week siege of eastern districts until opposition fighters broke through on Saturday.The push saw a coalition of rebels, Islamists and jihadists cut off the regime's own main access road on Aleppo's southern edges.The offensives have left residents reeling from skyrocketing prices and food shortages and afraid of further violence.Each side has 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 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 online analysis titled "Aleppo - The Mother of All Battles"."Although an opposition conquest of the whole city appears highly unlikely, breaking the siege has sent a powerful message of opposition indefatigability."Sporadic clashes hit the city's outskirts on Tuesday, but there were no signs 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 under way."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."Clashes were taking place in the southern suburbs, in the key district of Ramussa and a collection of military academies."Most of the clashes in Ramussa are taking place against Hezbollah and Iranian fighters," Abdulrahim said, referring to Bashar al-Assad's backers in the Lebanese Shiite movement and Tehran.The Syrian Observatory for Human Rights monitor also said there were clashes in Ramussa, adding that strikes on a rebel-held district killed nine civilians on Tuesday.