Kerry told reporters at the State Department Thursday that the facts are not complicated.
"Sarin was used. Sarin killed. The world can decide whether it was used by the regime which has used chemical weapons before, the regime which had the rockets and the weapons, or whether the opposition secretly went unnoticed into territory they don't control to fire rockets they don't have containing sarin that they don't possess to kill their own people," he said. "And that without even being noticed, they just dissembled it all and packed up and got out of the center of Damascus controlled by Assad. Please."
In an earlier interview with the U.S. network Fox News, Assad denied his forces launched the poison gas attack that killed hundreds near Damascus. He said he is fully committed to disposing of his government's chemical arsenal, and he promised to abide by a U.S.-Russia deal aimed at destroying the chemical stockpiles by the middle of 2014.
Assad described the situation as "complicated," saying destruction of the weapons would cost about $1 billion and would take a year or "maybe a little more."
More than end-date needed
American University international relations professor Sharon Weiner told VOA the success of the U.S.-Russian timetable will depend on both powers agreeing to more than just an end-date.
"In terms of destroying the weapons, can it be done in that period of time? Absolutely," she said. "But, the question is reaching agreement on how you are certain that you have all the weapons and that there is not some residual concern that you missed something.”
Weiner also said the ultimate cost of destroying Syria's stockpiles is unclear.
"It depends on how far you have to transport them and ... how much you have to pay to protect the people who are destroying them. I think anyone who tries to give a realistic estimation of the cost right now just doesn't have the information they need to do that,” she said.
Russian Defense Minister Sergei Shoigu said his country has no plans to destroy Syria's chemical weapons on its own territory, although he acknowledged it has the facilities to do so.
Russia and the United States are the only countries with the industrial scale capacity to handle mustard, VX, sarin or cyanide-armed munitions, but the import of chemical weapons is banned under U.S. law.
The disarmament plan, which is still being debated by U.N. Security Council envoys, requires Syria's government to turn over details of its chemical weapons by Saturday. Assad said he is willing to do this "tomorrow," and can provide experts access to the sites where the weapons are stored.
The Syrian leader criticized this week's U.N. report that confirms sarin nerve gas was used in an attack against civilians in the rebel-held suburb of Ghouta on August 21.
Although the report did not assign blame, the U.S. and other Western nations say it strongly suggested that government forces, not rebels, were responsible for the attack.
Assad called the findings "unrealistic," expressing doubt about the authenticity of the large amount of photos and videos purporting to show the aftermath of the attack.
U.N. Secretary-General Ban Ki-moon said evidence gathered by U.N. investigators in Syria and released Monday "indisputably" and "overwhelmingly" confirms the use of sarin on a relatively large scale in the attack on Ghouta.
The U.S. says the attack killed 1,400 people.
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