Humanitarian agency says the fight for Aleppo is 'one of the most devastating in modern times'

President Bashar al Assad's forces, with Russian air support, have now launched a counter-offensive.

Fighting for control of Syria's second city has intensified in recent weeks as rebel groups battling Syrian government forces have made gains.

International Committee of the Red Cross president Peter Maurer said hundreds of people have been killed and untold numbers injured, while most others are trapped without aid.

"No one and nowhere is safe," said Mr Maurer. "Shell-fire is constant, with houses, schools and hospitals all in the line of fire. People live in a state of fear. Children have been traumatized. The scale of the suffering is immense. For four years, the people of Aleppo have been devastated by brutal war, and it is only getting worse for them."

The remarks came as the White Helmets, a leading humanitarian aid group inside Syria, warned Sky News that more people will become radicalised if the fighting in Aleppo doesn't stop immediately.

Zedoun al Zoubi, whose staff were the first to enter the eastern side of the city after the siege of the rebel-held area was broken, said: "If this war continues the entire world will suffer. Years ago we've been telling, listen Syria is a dangerous country. Stop the war in Syria. No-one listens ... and now everyone suffers."

Mr Zoubi says all combatants must agree now to a ceasefire to allow aid into hard-to-reach areas and ease the civilian population's suffering.

Ten days ago the rebels partially broke the siege of Aleppo after they captured the district of Ramouseh to the south.

The Free Syrian Army says the success was down to co-operation with Jabhat Fateh al-Sham (JFS), which was formally known as the Nusra Front, or al Qaeda in Syria.

It is a further complication on the battlefield and for any political process as the jihadists appear to be gaining support on the ground.

Speaking to Sky News from inside Aleppo, FSA Commander Abu Qutayba, said: "The co-ordination between us and Jabhat Fateh al-Sham is what made the difference. The aim of this co-ordination is to liberate the whole area from the regime. No one knows what will happen in the coming days, but maybe we will see developments that bring JFS closer to the Syrian people."

President Bashar al Assad's forces, with Russian air support, have now launched a counter-offensive.

Rescue workers in rebel-held Aleppo have reported very heavy bombing in recent days, with significant casualties.

Ismail Abdullah, from the Syrian Civil Defence group, told Sky News that Russian and Syrian airstrikes were targeting civilian infrastructure.

"Aleppo is the most dangerous city all over the world," he said.

"The situation is actually so dangerous we hear bombing all the day. A lot of people are killed every day.  Russian intervention changed the game, we have a lot of bombing targeting civil defence centres, targeting hospitals, targeting schools, targeting markets."

Russia has denied hitting residential areas and hospitals.

It says it is battling extremists inside the country.

But what is clear is that a once-prosperous city is in pieces and the people living in the shadows of an endless conflict are desperate.

Mr Zoubi says the world must intervene and "stop the madness".

"Children, adults everybody is desensitised now. We don't feel anything now," he said.

"I saw it with my own eyes. In Aleppo there was a barrel bomb, there was a huge explosion and a child heard it and started dancing. As if it is a music for them now. Normal life means there is bombs. Normal life means there is a jet fighter in the sky. Normal life means there is an airstrike. Normal life means there is not food."