UN calls for aid as all-out war looms in Aleppo

The United Nations has called for urgent aid access to Syria's Aleppo as regime forces and rebel fighters prepare for an all-out battle for control of the devastated city.

Fears are growing for trapped civilians ahead of what is expected to be a major battle for Aleppo, Syria's second city and a focal point of the country's five-year civil war.

Rebel factions and President Bashar al-Assad's regime 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 the city, once Syria's main economic hub, 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.

UNICEF said children and families in Aleppo were facing "a catastrophic situation", with up to two million people without running water for four days after fighting damaged electricity networks needed to pump supplies.

Aleppo has been divided between a rebel-held east and regime-controlled west since fighting erupted in the city in mid-2012.

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.