After his escape from the besieged city of Homs, photographer Paul Conroy tells Channel 4 News: "It's systematic slaughter, not a war."
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Mr Conroy, a freelancer and filmmaker who was working for the Sunday Times in Syria, spoke to Jon Snow from his hospital bed in London.
He was smuggled across the Lebanese border on 28 February with wounds to his legs after being injured in an attack by Syrian troops in the Baba Amr district of Homs.
The 47-year-old told Channel 4 News: "It's systematic slaughter, not a war. It's a massacre and the world sits by and watches. Someone has to break the geopolitical stalemate that is allowing this regime to crush the humanity out of Syria.
It's a massacre and the world sits by and watches. Paul Conroy, photographer
"I think we left behind another unfolding tragedy that in 10-15 years the world will say, how did we sit back and let this happen?
"There's mass destruction - children, women, men huddled together, basically planning their last minutes … unparalleled, systematic destruction of civilians, property. Everything that they stand for is gone.
"People would come up to me and say 'where's our help?', and I had no answers. I should have an answer. There should be an answer to this in this age. Somebody needs to step up to the plate and give these people an answer."
Mr Conroy, from Totnes in Devon, was carried out of Homs on a stretcher in a nerve-racking 26-hour operation that cost the lives of 13 Syrian volunteers.
'Laid their lives down'
He paid tribute to those who helped him escape, saying: "Those people laid their lives down for us and I must honour that level of commitment but saying what I can about what I saw. I salute them, they're heroes to the last."
The father-of-three had to leave behind the body of Sunday Times reporter Marie Colvin, who was killed in the attack. "It was one of the hardest decisions I've ever had to make. But Marie would have wanted the story over anything, and the only way I could honour her memory was to come out and tell the story.
"The world has lost one of its greatest observers and it will be a worst place without her, but I can justify leaving Marie to tell the story. I hope we honour her death by doing something. The time for talking is long gone. People are dying as we talk."
The International Committee of the Red Cross (ICRC) said on Friday that the Syrian authorities had handed over Ms Colvin's body along with that of the French photographer Remi Ochlik. The ICRC said it was taking their bodies by ambulance from Homs to the Syrian capital Damascus.