BEIRUT (AFP) - Residents in Syria's battleground city of Aleppo cowered indoors Saturday (Sept 24) as fierce air strikes toppled buildings and killed at least 45 civilians, after diplomatic efforts to revive a ceasefire failed.

Nearly two million civilians were left without water in the devastated northern city after regime bombardment damaged a pumping station and rebels shut down another in retaliation, the United Nations said.

Syria's Foreign Minister Walid Muallem told the UN General Assembly the Syrian army was making "great strides" in the conflict, with help from "true friends" Russia, Iran and Lebanon's Hezbollah.

Rebel-held districts in east Aleppo came under intense air and artillery fire for a fifth night on Friday as the army prepared a ground offensive to recapture the whole of the divided city.

Syria's main opposition coalition denounced the "silence of the international community", saying Damascus and its Russian allies were committing "a crime" in Aleppo.

Muallem told the UN: "Our belief in victory is even greater now that the Syrian Arab Army is making great strides in its war against terrorism, with the support of the true friends of the Syrian people, notably the Russian Federation, Iran and the Lebanese national resistance."

Saturday's death toll of 45 in Aleppo city was expected to rise because people remained trapped, said the Syrian Observatory for Human Rights, a Britain-based monitoring group.

It said another seven people were killed elsewhere in Aleppo province on Saturday.

"We were home when a missile crashed into our road," said one resident of the Bab al-Nayrab district who gave his name as Nizar.

"Half of the building just caved in and our baby was hit on the head. He died on the spot," Nizar said, the body of his son on the ground wrapped in a blanket.

Seven people were killed in a strike as they queued to buy yoghurt at a market in the Bustan al-Qasr district on the front line dividing the government-held west from the rebel-held east of the city.

The attack left a pool of blood and body parts strewn across the site, said an AFP correspondent on the scene.

Medics said they were carrying out many amputations to try to save the wounded, while supplies of blood and IV drips were running out.

On Friday, at least 47 people were killed in heavy bombing, among them seven children, the Observatory said.

There was massive destruction in several neighbourhoods, including Al-Kalasseh and Bustan al-Qasr, where some streets were almost erased by the bombardment.

Residents and activists said one type of bomb had produced earthquake-like tremors upon impact, razing buildings right down to their basements where many residents desperately seek safety during attacks.

The civil defence organisation known as the White Helmets was overwhelmed by the scale of the destruction, particularly after several of its bases were damaged by bombing on Friday.

With no electricity or fuel for generators, the streets of Aleppo are pitch black and difficult to navigate at night, and the fuel shortage has also made it tough to fill up vehicles.

In many places, rubble has rendered streets impassable, effectively sealing off neighbourhoods.

The UN children's agency Unicef said the loss of mains water posed serious health risks in rebel-held areas as the only alternative source of drinking water was from highly contaminated wells.

"It is critical for children's survival that all parties to the conflict stop attacks on water infrastructure," it said.

Further south in the central city of Homs, a convoy of 36 aid trucks reached the rebel-held district of Waer, the International Committee of the Red Cross said.

The denial of access to food, water and medicines has been used repeatedly as a weapon by all sides in the five-year war, which has cost more than 300,000 lives and displaced over half the population.

The approximately 250,000 people in east Aleppo have been under near-continuous siege since government troops encircled the area in mid-July.

A truce deal negotiated between Moscow and Washington brought a few days of respite in Aleppo earlier this month, but no humanitarian aid.

The deal has since fallen apart, and on Thursday the Syrian army announced an operation to retake all of Aleppo.

US Secretary of State John Kerry met his Russian counterpart Sergei Lavrov again in New York on Friday to try to restore the ceasefire, but without success.

Kerry said they had made "a little bit of progress" on resolving their differences.

But Lavrov said that it would be "senseless" to impose a new truce because the United States had failed to separate moderate rebel groups from jihadists.

Muallem also told the UN that a US-led coalition air strike that killed at least 62 Syrian soldiers a week ago was intentional "and not an error, even if the United States claims otherwise".