More than 500 churches around the world are ringing their bells every day in tribute to the thousands of civilians being slaughtered in Aleppo.

The Syrian city has been under brutal siege for weeks, intensified by Russian and Syrian bombing of rebel-held areas that began last month. Around 275,000 people are believed to be trapped in the east of the city.

The Lutheran Kallio church in Helsinki began the tolling on October 12. Vicar Teemu Laajasalo told Reuters: "After reading the news that day, of Aleppo, we decided to ring funeral bells at 5pm to remember the deaths. First I just asked a few local churches to join us."

They have now been joined by hundreds of churches nationwide and across the globe, including congregations in Britain, America and Australia.

Each church will ring the bells daily in the lead up to United Nations Day on October 24.

"We all see the devastating events in Aleppo, but at the same time feel powerless to help, because of the complexity of the situation. With the bells, we want to make our voices heard, and give hope for better future," Laajasalo said.

Uppsala Cathedral in Sweden is one of the churches to join in the initiative. Archbishop of Sweden Antje Jackelén and cathedral dean Annica Anderbrant said in a joint statement: "We become desperate and helpless when we learn of the stories about the situation in Aleppo and other places in Syria – the fate of the children, hospitals in impossible working situations, innocent civilians who suffer.

"When impotence is greatest, it is important to gather in prayer."

Zeid Ra'ad al Hussein, the UN high commissioner for human rights, today branded the bombardment of Aleppo "crimes of historic proportions".

In a speech before the UN Human Rights Council, he said the "indiscriminate airstrikes across the eastern part of the city by government forces and their allies are responsible for the overwhelming majority of civilian casualties".

He accused those forces of committing "war crimes" and added: "If knowingly committed as part of a widespread or systematic attack directed against civilians, they constitute crimes against humanity."

The "collective failure of the international community to protect civilians and halt this bloodshed should haunt every one of us", he said.

To sign up your church for Bells for Aleppo, click here.