At least 23 civilians have been killed in a single day in tit-for-tat attacks by government forces, Russia and anti-government fighters in Syria's Aleppo, according to a monitoring group.

Air strikes by unidentified jets killed at least 12 civilians on Friday in the opposition-held districts on the northern edge of eastern Aleppo, said the UK-based Syrian Observatory for Human Rights.

The bombardment, believed to have been carried out by Syrian government jets and those of allied Russia, has worsened the already dire humanitarian situation in eastern Aleppo.

Baby girl pulled out alive from rubble in Syria's Idlib

The Syrian observatory said that local hospitals cannot cope with the increasing numbers of injured amid a shortage in medical equipment and supplies.

Eleven other civilians were killed in shelling attacks by fighters against government-controlled areas in Aleppo on Friday, said the observatory,  that relies on a network of activists inside Syria.

In Geneva, the UN Human Rights Council denounced the ongoing attacks on civilians in eastern Aleppo, in a resolution that was opposed by Russia.

The resolution text "strongly condemns the military offensive on eastern Aleppo being conducted by forces loyal to the Syrian authorities, and calls on them to halt immediately the indiscriminate bombing of the civilian population".

The decision that was drafted by Western and Arab countries was adopted with 26 "yes" votes.

Plight of the wounded

For its part, the World Health Organization repeated its urgent demand on the rival sides to allow the evacuation of sick and wounded people from eastern Aleppo.

More than 840 people have been injured in this besieged part of the divided city over the past week, nearly a third of them children, according to the UN aid agency.

READ MORE: Letter from Aleppo - 'My city is not just a death toll'

WHO also demanded that medical aid should be let in, and that attacks on hospitals stop.

"Blocking whole populations from access to medical care, food and water is intolerable. It is inexcusable cruelty," said Pete Salama, who heads WHO's medical emergencies programme.

An estimated 300,000 people in eastern Aleppo are believed to be under siege by  government forces.

Aleppo onslaught: Volunteer rescuers struggle to save the injured

Friday also marked the first anniversary of Russia's air campaign in Syria.

More than 9,000 people, including nearly 4,000 civilians, have been killed in one year of Russian air raids in Syria, according to the Syrian observatory.

It said the death toll included about 3,800 civilians and 5,500 fighters from the Islamic State of Iraq and the Levant (ISIL) group and various rebel factions.

A further 20,000 civilians have been wounded in Russia’s year-long offensive of air strikes, it said.

Later on Friday, the Russian government rejected the figures provided by the observatory.

"We do not consider information about what happens in Syria from an organisation based in Britain to be reliable," Dmitry Peskov, Kremlin spokesman, said

The one-year anniversary of Russia’s military intervention came a day after Russia said it would press on with its bombing campaign in Syria, ignoring a threat by the US to suspend its engagement.  

Syria war: What is the importance of Aleppo?

Source: Al Jazeera News And News Agencies