Former foreign secretary David Miliband has warned of "house-to-house murder" being carried out in war-torn Aleppo.
Mr Miliband, president of the US-based charity International Rescue, which still has aid workers in the Syrian city, expressed fears that the carnage would spread to Idlib next.
"The truth is there's chaos, people are looking to find their families, and people are reporting, some of them, that the ceasefire is holding, others that there is no safety at all, and it's sheer terror.
"And that is the only way to describe this because people are fleeing from appalling bombardment, and house-to-house murder that's being documented by the UN, and they don't know if it's going to follow them from Aleppo to Idlib," he told the BBC Radio 4 Today programme.
Mr Miliband also criticised the 2013 Commons vote against then prime minister David Cameron's bid to back US military intervention after the use of chemical weapons by the Syrian regime.
Opposition to Mr Cameron's stance was led by then Labour leader Ed Miliband.
Though the ex-foreign secretary did not mention his brother by name, he said the Commons decision had contributed to a vacuum of Western leadership.
"I think that was quite important. The red line over chemical weapons needed to mean something to uphold international norms, never mind American credibility and support.
"Secondly, I think that the vote, which after all, I think I'm right in saying, 560 MPs voted to support military action under certain conditions, they just couldn't agree with each other as to what those conditions should be.
"And it's sowed a deal of confusion that I think, undoubtedly, contributed to the sense of vacuum that has been discussed and described this week."