Baroness Amos: 'Strategic opportunity missed' in Syria
15 March 2016 Last updated at 20:49 GMT
Baroness Valerie Amos says the international community made a "terrible mistake" in 2012, when it dismissed a Russian offer to force Syrian President Assad to step aside as part of a peace deal.
She was UN under-secretary-general for humanitarian affairs from 2010-2015.
Speaking to BBC Newsnight's Evan Davis, Baroness Amos described it as a "strategic opportunity that was missed".
You can watch the full interview at 22:30 GMT on BBC Two or catch up afterwards on iPlayer (UK only)