Syrian forces launched a major ground offensive on a rebel-held district of Aleppo, the biggest assault yet in a new campaign aimed at wiping out rebel forces and retaking a city that is key to ending the five-year war.

In the early hours of Wednesday, air strikes hit a hospital in a rebel-held area, putting it out of service.

"The warplane flew over us and directly started dropping its missiles on this hospital ... at around 4am [local time]," Mohammad Abu Rajab, a radiologist at the M10 hospital, told Reuters news agency.

"The rubble fell in on the patients in the intensive care unit."

On Tuesday, Syrian state TV said that troops had captured Farafra district, near Aleppo's famous citadel, and fighting was under way near the historic core of the northern city. 

A military official told AFP news agency that government forces "retook control of all of the Farafra district" and were now "demining the area".

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The official said the advance "comes as a continuation of the military operation that was announced that includes an aerial component and an artillery and ground component". 

Aleppo has endured the worst aerial onslaught since the start of the war in 2011 with more than 400 people killed, hundreds of others wounded, and buildings flattened since a short-lived ceasefire broke down last week. 

Free Syrian Army (FSA) fighters told Al Jazeera that pro-government forces, including Shia militias, carried out "a ground push" in the Bab al-Antakya district.

"They [rebels] said that they repelled that attack and killed a number of pro-government militia" during fighting that lasted a couple of hours, said Al Jazeera's Charles Stratford, reporting from across the Turkish border in Gaziantep.

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Stratford said a website run by the FSA reported two attacks in which 11 pro-government militiamen and Syrian soldiers were killed by rebel fighters, "keeping them from moving forward".

It was not possible to independently confirm any of the claims. 

In the district of Handarat, north of Aleppo, government forces also advanced against the rebels, he said.

Government fighter jets backed by Russia's air force also continued to target the city and its outskirts. Video obtained by Al Jazeera showed the latest air strikes on Tuesday with rescue crews rushing to the scene to pull people from under the rubble.

Residents said bunker-busting bombs were dropped in the al-Shaar neighbourhood killing 24 civilians, including 15 people from the same family. 

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The historic quarter of Aleppo, one of the world's oldest cities, is home to the Umayyad Mosque, a UNESCO world heritage site. The 11th-century minaret of the famed mosque collapsed in April 2013 during fighting.

The Britain-based Syrian Observatory for Human Rights, which relies on a network of activists inside Syria, confirmed that government forces were advancing on the old quarter.

"There was intense shelling earlier. It seemed the [government] was preparing for the attack," said Ibrahim Alhaj, a member of the Syrian Civil Defence, volunteer first-responders also known as the "White Helmets".

He added news from the front line suggests a large mobilisation of pro-government militias in the old city.

The burned sign of the emergency section of a field hospital is pictured after an air strike in the rebel-held al-Maadi neighbourhood of Aleppo [Reuters/Abdalrhman Ismail]

Speaking on condition of anonymity, a Syrian military official in Damascus told the AP news agency that operations in Aleppo will continue until "terrorists" in the eastern parts of the city are "wiped out".

If President Bashar al-Assad's forces do recapture the ancient city it would mark a major turning point in the war, which has killed an estimated 400,000 people and displaced millions.

"It will be a big victory for the regime if it takes Aleppo," opposition activist Mahmoud al-Basha told Al Jazeera. "Aleppo is the biggest stronghold for FSA. There are no more cities for the FSA in Syria - they lost Homs, there is no FSA in Hama and Damascus."

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Syria's army announced an operation to retake the opposition-held east of Aleppo city following the collapse of the Russia-US orchestrated truce. Al Jazeera's Stratford said people he spoke to "are literally terrified" at the prospect of a ground operation.

"They feel very helpless. They have completely given up on any kind of diplomatic effort. They have completely lost faith in what the international community is saying," he said. 

At least 250,000 people still live in the besieged part of Aleppo. 

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Source: Al Jazeera News And Agencies