(CNN) At least 26 people have been killed, including six children, in airstrikes from Monday, which have targeted rebel-held areas of the northern Syrian city of Aleppo, according to the UK-based monitoring group Syrian Observatory for Human Rights (SOHR).

Nine other people were wounded, some in critical condition, SOHR said. CNN has not been able to independently verify such claims.

It wasn't clear who conducted the airstrikes, but the opposition and the US have blamed the Syrian regime and Russian warplanes for such attacks since a ceasefire broke down.

SOHR claimed rebel-held areas in the eastern part of the city had been struck by barrel bombs Tuesday, as fighting between regime and rebel forces continues throughout the city. Throughout the conflict the crude explosives have been the calling card of Syrian government forces.

Syria: The biggest issue for the next US president?

Syria: The biggest issue for the next US president? 00:41

Syria: The biggest issue for the next US president?

The bombardment came after a bloody weekend which saw the deaths of 85 people and injuries to 300 others, and a day after the UN Security Council met in a special crisis meeting.

In that meeting, Secretary-General Ban Ki-moon and the US Ambassador to the UN, Samantha Power, separately described the Syrian regime's brutal offensive against areas of the besieged northern city of Aleppo as "barbaric."

Following the collapse of a short-lived, US-and Russia-brokered ceasefire, Syrian forces pounded rebel-held eastern Aleppo on Sunday.

UN officials: Attacks 'barbaric'

Ban said he was "appalled" by the military escalation in the beleaguered Syrian city and that the use of bunker-busting bombs "brings the violence to new depths of barbarity."

What happened to the ceasefire? • Monday, September 12: The US-Russia brokered ceasefire starts at sunset • Minor violations mar the ceasefire's early hours. • By Thursday, September 15, Russia, US are accusing each other of not fulfilling their obligations. • On Friday, UK-based Syrian Observatory for Human Rights reports civilian deaths from airstrikes in Idlib, then Sunday in Aleppo. • On Saturday, the US conducts an airstrike which kills Syrian forces. The Pentagon says it was accidental, and US officials express regret. Russia's ambassador to the United Nations calls the strike "an extraordinary display of American heavy-handedness." • On Monday, September 19, a UN aid convoy's attacked outside Aleppo. The US blames Russia. Russia says it was rebel forces. It all but ends hope of an extended ceasefire.

Ban said that the airstrikes, incendiary weapons and bombs in densely populated areas may amount to war crimes.

Power echoed Ban's language, saying that Russian support of Syrian President Bashar-al Assad's deadly offensive was "barbarism."

"What Russia is sponsoring and doing is not counterterrorism, it is barbarism," she told the Security Council.

Special Envoy for Syria Staffan de Mistura called the recent days "chilling." He said the "past week has been one of the worst ones in Syria during the near six years of this devastating conflict."

Walkout

On Sunday, Matthew Rycroft, British ambassador to the UN, staged a walk-out at an emergency session of the UN Security Council, along with his French and US counterparts, to protest against the Syrian regime's latest offensive in Aleppo.

Rycroft told the Security Council meeting that it was "difficult to deny" that al- Assad and his Russian allies were engaged in committing war crimes.

We need to decide how to stop this. Now. #Syria's @UN ambassador is not interested in that decision. That's why I just walked out of #UNSC. — Matthew Rycroft (@MatthewRycroft1) September 25, 2016

"After five years of conflict, you might think that the regime has had its fill of barbarity -- that its sick bloodlust against its own people has finally run its course," he said.

"But this weekend, the regime and Russia have instead plunged to new depths and unleashed a new hell on Aleppo."

'This isn't Pompeii'

As Aleppo was hammered by airstrikes , activists and aid workers posted dozens of pictures and videos online. Each of them heartbreaking, each of them helping to convey horror of the besieged city.

On Friday afternoon, a photo of a father and son who had fallen victim to the war emerged. Covered in blood and dust, the pressure of the rubble holding their final postures in place.

Photos: Heartbreaking image following Aleppo airstrike Hide Caption 1 of 7 Photos: Heartbreaking image following Aleppo airstrike Brahim Sawas and his 10-year- old son, Mahmoud, fell victim to the war in Syria on Friday, September 23, in eastern Aleppo. They were covered in blood and dust, the pressure of rubble holding their final postures in place. Hide Caption 2 of 7 Photos: Heartbreaking image following Aleppo airstrike Volunteers work to recover the bodies of the father and son. Hide Caption 3 of 7 Photos: Heartbreaking image following Aleppo airstrike Mahmoud Sawas' uncle holds the boy's body after the deadly airstrike. Hide Caption 4 of 7 Photos: Heartbreaking image following Aleppo airstrike Mahmoud Sawas' father and younger brother also died in the strike. Hide Caption 5 of 7 Photos: Heartbreaking image following Aleppo airstrike The Syrian Civil Defense, also known as the White Helmets, work to save 8-month-old Muhammad Sawas from under the rubble. But it was too late. Hide Caption 6 of 7 Photos: Heartbreaking image following Aleppo airstrike Muhammad Sawas' devastated mother cradles his lifeless body. Hide Caption 7 of 7

Hospitals overwhelmed

The bombardment destroyed residential centers and overwhelmed hospitals.

"Everyone in Aleppo is depressed," an activist on the ground told CNN.

"They don't know what they have done to become targets for warplanes. Fear is clear in the eyes of anyone you see walking the streets of Aleppo. Yesterday I saw a woman walking on the street and crying, no clear reason, just crying."

Hundreds of airstrikes have pummeled the city, home to more than 250,000 people, since the Syrian government, backed by Russia, announced a renewed, "comprehensive" offensive Thursday.

Only 20 doctors remain in eastern Aleppo, the activist added.

Transfers

On Monday, the Syrian government began transferring around 350 people from the besieged rebel-held neighborhood of al-Waer in the city of Homs as part of a deal the regime calls "national reconciliation" but that the opposition has referred to as a "starve or surrender" policy.

According to the state-run SANA news agency, the transfer of 120 armed rebels and their families by bus from al-Waer to an area in the northwestern countryside of Homs province is the third phase of the deal which was struck with rebels in December.

The anti-regime activist group Homs Media Center said the number of armed men was 100.

"The agreement aims to evacuate all gunmen and remove weapons from al-Waer neighborhood in a step towards the return of all the state institutions to it," SANA reported.

The governor of Homs, Talal Barazi, is quoted as saying that 41 gunmen from the neighborhood recently turned themselves into authorities and handed over their weapons to the regime as part of a government amnesty.