UNICEF says nearly two million people without water as jets continue pounding Aleppo ahead of ground offensive.
Warplanes mounted a new wave of heavy air strikes on rebel-held areas of Aleppo on Saturday pressing on with a major offensive by the Russian-backed Syrian military to take back the entire city.
The attacks came as a UN official said nearly two million people in Aleppo are without running water as security conditions deteriorate.
Residents of rebel-held eastern Aleppo say it has been subjected to the most ferocious bombardment of the war since the government declared a new offensive that has killed dozens of people in the last three days.
"Unfortunately it continues. There are planes in the sky now," Ammar al-Selmo, head of Civil Defence in the opposition-held east, told Reuters news agency.
Rebels said air strikes on Saturday morning hit at least four areas of the opposition-held east, home to more than 250,000 people. The attack has drawn on ordinance more destructive than anything previously used against the area, and many buildings have been entirely destroyed, residents say.
"They are using weapons that appear to be specifically for [bringing down] buildings," said a senior official in an Aleppo-based rebel faction, the Levant Front. "Most of the victims are under the rubble because more than half the civil defence has been forced out of service."
The Syrian Observatory for Human Rights, a British-based organisation that reports on the war, said at least 50 people had been killed on Saturday.
"The raids are intense and continuous," said Syrian Observatory director Rami Abdulrahman.
The Syrian army said it is targeting rebel positions in the city and denied hitting civilians.
Water sources cut
Hanaa Singer, UNICEF representative in Syria, said intense attacks damaged the Bab al-Nairab station that supplies some 250,000 people in rebel-held eastern parts of the contested city with water.
Singer said in retaliation, the Suleiman al-Halabi pumping station, also located in the rebel-held east, was switched off - cutting water to 1.5 million people in government-held western parts of the city.
"Depriving children of water puts them at risk of catastrophic outbreaks of water-borne diseases," Singer warned in her statement released late Friday.
Residents say more than 150 air strikes have been launched, crushing emergency service structures, as well as underground shelters used by civilians to hide from bombings.
At least 30 neighbourhoods were targeted, Al Jazeera’s Amr al-Halabi said, reporting from Aleppo. He added the relentless strikes hampered the ability of rescue workers to help civilians caught up in the fighting.
Three centres for a volunteer rescue group known as the White Helmets were also hit.
"We have four centres in eastern Aleppo. The aircraft targeted three centres. Two of them are now out of service," Abdul Rahman al-Hassani of the White Helmets told Al Jazeera.
He added five vehicles belonging to the group were destroyed, including an ambulance.
"Our centres were the direct target [of the strikes]," Hassani said.
"We feel the earth trembling and shaking under our feet. Aleppo is burning," said Bahaa al-Halabi, an activist from a besieged rebel-held district.
The Syrian military, which is backed by the Russian air force, said it was starting a new operation against the rebel-held east.
The Syrian defence ministry has called on residents to move to government-held areas, adding there would be "no detention, or inquiry to any citizen" who reached the checkpoints that divide the city.
A high-ranking military source confirmed the intense bombardment was in preparation for a ground assault.
"We have begun reconnaissance, aerial and artillery bombardment," he told the AFP news agency, adding the strikes could "go on for days" before the ground operation starts.
Aleppo was once Syria’s commercial and industrial hub but has been ravaged by fighting and roughly divided between government control in the west and rebel control in the east since mid-2012.
The announcement of the new Syrian army offensive on Thursday came as international powers failed to revive a collapsed ceasefire during diplomatic talks in New York.
The Syrian war has seen more than 400,000 people killed and almost 11 million Syrians - half the country’s prewar population - displaced.