US Secretary of State John Kerry said Thursday that the US knows that the Assad regime possesses sarin gas, and that there’s not a shred of evidence that the opposition does. Sarin is believed to have been used in the attack on a Damascus suburb on August 21 in which over 1,400 people were killed, according to US figures.

“There’s no indication – none – that the opposition is in possession [of the gas] or has launched a CW variant of these [122 -millimeter] rockets,” Kerry said at a State Department press briefing Thursday.

Russia has consistently maintained that the rebels are behind that chemical attack and that it will present proof of such to the UN Security Council.

“This isn’t complicated. When we say we know what is true, we meant it. We have a definitive UN report strengthening case,” Kerry said.

“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. And then without even being noticed, they just disassembled it all and packed up and got out of the center of Damascus, controlled by Assad,” said the secretary of state.

He stressed that the UN Security Council must be prepared to act next week on Syria, that it must stand up and speak out against the use of chemical weapons.

“Time is short. Let’s not spend time debating what we already know. We have to recognize that world is watching to see whether we can avoid military action, A complete removal of the chemical weapons is possible through peaceful means.”

The Russian-US agreement reached last week, which aims to have Syria’s WMD cache destroyed by mid-2014, “clearly said it must be enforceable, it must be as soon as possible, it must be real,” said Kerry

“We need a binding resolution to achieve that goal. We must make the Geneva deal meaningful. We have an important goal in New York and we have to achieve it as rapidly as possible,” said the secretary of state.

“This fight about Syria’s chemical weapon is not a game. It’s real. It’s important that this Geneva agreement be enforced,” he stressed.

On Tuesday, US Ambassador to the United Nations Samantha Power also said that the UN report points to Assad.

“The technical details of the UN report make clear that only the regime could have carried out this large-scale chemical weapons attack.”

“One type of munition, 122mm rockets, were found to have been used in the attack, a type associated with “previous regime attacks,” Power said, adding that “we have not observed the opposition manufacturing or using this style of rocket.”