Foreign Minister Julie Bishop has warned the United States and Russia must urgently broker a peace deal to stop the bloodletting in Syria following "appalling" reports of mass atrocities in Aleppo.

A ceasefire in the devastated city, finalised overnight, has been undermined by what the United Nations said were credible reports that pro-Syrian forces had executed dozens of civilians as they evacuated the former rebel stronghold.

Foreign Minister Julie Bishop has urged President Donald Trump to remain engaged in the region. Credit:Andrew Meares

Ms Bishop said the claims must be investigated and a political solution was the only way to end the civil war, which has now raged for six years and has killed hundreds of thousands of people.

"This is one of the worst humanitarian disasters that we have witnessed in many, many years," she said. "There are reports of mass atrocities. They must be investigated. This is an appalling humanitarian disaster and the conflict has to end."