The letter is signed by Ahmed Moaz al-Khatib, the president of the Syrian National Coalition, an umbrella body of anti-Assad rebels.
Deadly explosion hits rebel city
Violence continued to plague the country this week, as Syria marked the second anniversary of the war. At least 62 people died across the nation Wednesday.
Also on Wednesday, a massive explosion ripped through the city of Raqqa, two days after rebels claim to have captured this provincial capital.
"Honestly, I cried when I saw the bodies. It's the first time I witnessed such ugly images," said Ahmed Tayar, a writer and resident of Raqqa who saw the aftermath of the mysterious blast.
Tayar, 50, told CNN he saw 19 burned bodies at the scene of the blast.
An uncut, six-and-a-half-minute amateur video of the immediate aftermath of the explosion showed panicked rebel fighters and civilians racing to pick up several wounded and apparently dead men.
In the video, two middle-age men lay bleeding in the street next to scattered oranges and tomatoes that they appeared to have been carrying. Smoke billowed in the distance from a burning car. The entire facade of a nearby three-story building appeared to have been blown off by the force of the blast.
It's not clear what caused the explosion, but supporters of the opposition were quick to blame the Syrian government for the blast.
"I thinks it's the regime who killed those people, and that is because the regime wants to take revenge against Raqqa because that it's the first province that was fully liberated," Tayar told CNN.
On Tuesday, eyewitnesses told CNN residents were fleeing rebel-controlled Raqqa, as government war planes bombed targets in the city.
"While we were there, there were two MiG fighter jets running above us that were throwing down missiles," said Ibrahim Olabi, an activist with the Aleppo Media Center, after he visited Raqqa on Tuesday.
"There were no ground clashes," he added, "but only airstrikes from above."