American Secretary of State John Kerry says it is essential a Russia-US deal on eradicating Syria's chemical weapons arsenal be enforced and that the United Nations Security Council act on it next week.
Russia and the United States brokered the deal last week to avoid possible US military strikes.
Under the deal, Syrian President Bashar al-Assad would account for his chemical weapons within a week and see them destroyed by the middle of next year.
Mr Kerry said there was no doubt whatsoever that sarin was used in Syria in an attack in Damascus that killed about 1400 people.
He said a UN report made it clear the Syrian government carried out the attack.
Mr Kerry said it is vital for the international community to stand up and speak out in the strongest possible terms about the importance of enforcible action to rid the world of Syria's chemical weapons, Reuters reports.
Russian President Vladimir Putin insists it is still not proven that the Syrian Government was behind the chemical weapons attack.
Russia holds the power of veto on the Security Council.
Mr Putin says he is confident, but not 100% certain, the Syrian Government will carry out a commitment to destroy its chemical weapons.