The bombardment of Aleppo in Syria is "demonic" and shows "absolute contempt for the human spirit," Justin Welby has said.

Speaking during his visit to the Rome this week, the Archbishop of Canterbury added that the Syrian conflict is "as bad as anything we've seen in the last century – and there have been terrible atrocities".

In an interview with ITV News, Welby said: "What is being done is evil, it is demonic, it is the absolute contempt for the human spirit, for the dignity of the human being. It is the brushing aside of the poor and the weak and the fragile, in a way that is as bad as anything we've seen in the last century."

Asked whether the UN had failed to bring peace to Syria, Welby said that it had and stressed the organisation's limitations. He said: "One has to ask: what could they have done? The UN has no army. The UN has done what it could do. The UN is a fallible and weak institution. Of course it is, it's human. The UN is where you bring the worst hatreds in the world, and you put them in a room and see if you can make some progress. Sometimes it does – on this occasion it hasn't."

Welby added: "Has it failed? Yes, of course it's failed but we've all failed."

The Archbishop called on the Syrian President, Bashar al-Assad, to take the "heroic" step of standing down.

"His stepping aside would be the most heroic thing he could do in his life and the best decision he'd ever taken," Welby said.

The UN estimates that 275,000 people are currently trapped in a siege in Aleppo, with Syrian pro-government forces attacking the city from the south.

Assad's government, with Russian air support and Iranian ground forces, launched the assault on Aleppo last month, a week into a ceasefire agreed by Washington and Moscow.

The US and other Western countries say Moscow and Damascus are guilty of war crimes for deliberately targeting civilians, hospitals and aid deliveries to crush the will of the people trapped under siege. The Syrian and Russian governments claim that they target only militants.

The collapse of the ceasefire last month left US policy on Syria in tatters and was a blow for US Secretary of State John Kerry, who negotiated the truce over months of intense diplomacy with Moscow despite scepticism in Washington, including from other senior figures in the Barack Obama administration.

Last month the Republican Senator John McCain called Kerry "intrepid but deluded" for relying on Moscow.

Kerry suspended talks with Russia on Monday, accusing Moscow of failing to live up to its commitments to halt fighting and assure aid reached besieged communities.

"We are not giving up on the Syrian people and we are not abandoning the pursuit of peace," Kerry said on Tuesday in a speech in Brussels.

He accused Moscow of turning a blind eye to Syria's use of poison gas and "barrel bombs" – oil drums packed with explosives – to kill civilians.

Additional reporting by Reuters