Holocaust survivors have called on the international community to indict those responsible for mass deaths in Syria, and to act immediately to end the violence there.

The atrocities must stop now, a Canada-based group of octogenarians and nonagenarians told a crowd in Toronto on Wednesday.

Howard Chandler, 88, said aspects of the situation in Syria today reminded him of what he experienced in Poland during the Second World War.

"We are asking the international community to please intervene before it is too late," he told CBC News.

"Today when I turn on the news I see once again that the world is standing by," he said. "Scenes from Aleppo in Syria are sickening."

He added: “People who commit crimes have to know that, at the end of the day, if they're still alive, they will have to account for their activities."

Mr Chandler was 10 and living in Poland in 1939 when the Nazis occupied the country, and murdered almost his entire family. Only he and his brother were left alive when the Second World War ended in 1945.

Thousands of fighters and civilians have left Aleppo and the surrounding area in buses this week, after Syrian government forces and their supporters took control of the rebel-held areas of Aleppo this month, razing entire neighbourhoods.

The evacuation had stalled after rebel forces said pro-government militias fired on a convoy carrying evacuees and robbed them.

About half a million people have been killed since the start of Syria's civil war five years ago.

According to the UN High Commissioner for Refugees, 4.8 million people have fled the country, and about 74,000 refugee children are living without their fathers or both parents.

Vera Schiff said watching the scenes in Aleppo on television make her cry.

"After my liberation in 1945, I thought the world would have learned its lesson yet here we are again watching news from Aleppo," said the 90-year-old, who was sent to a concentration camp when she was 15. She spent four years detained there and lost every member of her family.

“You see those pictures come out of there with all this heartbreak with those maimed and crying children. It breaks my heart because I can see the replay of those children, of our children which were lined up to be gassed in the camps and the mothers who could not save them,” Ms Schiff said.

Aleppo before the Syrian Civil War Show all 12 1 /12 Aleppo before the Syrian Civil War Aleppo before the Syrian Civil War Aleppo before the Syrian Civil War A man crosses a street in Aleppo, December 12, 2009 Reuters Aleppo before the Syrian Civil War Aleppo before the Syrian Civil War A vendor sits inside an antique shop in al-Jdeideh neighbourhood, in the Old City of Aleppo, December 12, 2009 Reuters Aleppo before the Syrian Civil War Aleppo before the Syrian Civil War A view shows part of Aleppo's historic citadel, overlooking Aleppo city, Syria Reuters Aleppo before the Syrian Civil War Aleppo before the Syrian Civil War A view shows part of Aleppo's historic citadel, Syria Reuters Aleppo before the Syrian Civil War Aleppo before the Syrian Civil War Visitors walk inside Aleppo's Umayyad mosque, Syria Reuters Aleppo before the Syrian Civil War Aleppo before the Syrian Civil War People walk inside the Khan al-Shounah market, in the Old City of Aleppo, Syria Reuters Aleppo before the Syrian Civil War Aleppo before the Syrian Civil War A man walks past shops in al-Jdeideh neighbourhood, in the Old City of Aleppo, Syria Reuters Aleppo before the Syrian Civil War Aleppo before the Syrian Civil War People walk along an alley in al-Jdeideh neighbourhood, in the Old City of Aleppo, Syria Reuters Aleppo before the Syrian Civil War Aleppo before the Syrian Civil War Visitors tour Aleppo's historic citadel, Syria December 11, 2009 Reuters Aleppo before the Syrian Civil War Aleppo before the Syrian Civil War A general view shows the Old City of Aleppo as seen from Aleppo's historic citadel, Syria December 11, 2009 Reuters Aleppo before the Syrian Civil War Aleppo before the Syrian Civil War People walk near Aleppo's Bab al-Faraj Clock Tower, Syria October 6, 2010 Reuters Aleppo before the Syrian Civil War Aleppo before the Syrian Civil War A man stands inside Aleppo's historic citadel, overlooking Aleppo city, Syria December 11, 2009 Reuters

She added that she feels it is her duty to speak out to try to prevent further atrocities against innocent civilians.

“That this is still allowed to happen, that people look at horrible pictures, shrug their shoulders and go about their day — it's not permissible," she said.

Gerda Frieberg, 91, said she knows what it is like to be all alone, like the children orphaned in Syria.

“After what we went through, we thought the world would learn something, but I see they didn't," she said. "That's the worst that could happen”.

Avi Benlolo, president and CEO of the Friends of Simon Wiesenthal Centre for Holocaust Studies, which invited the survivors to speak, said the atrocities must stop.

“If we don't speak out, and speak out forcefully, against these atrocities I'm concerned it's going to become normalised,” Ms Benlolo said.