"If you have information about ceasefire violations, you can contact any member of the International Syria Support Group. In the US, you can contact a special coordinating group monitoring the Syrian ceasefire in Washington," Ratney said in an Arabic-language statement distributed by the Syrian opposition's High Negotiations Committee.
The special envoy stressed that the use force in a ceasefire regime is allowed "only when there is an imminent threat or in case of self-defense."
Ratney called on the sides to the truce to provide immediate humanitarian access to areas under their control to all those in need.
Russia and the United States reached an agreement on the ceasefire in Syria on February 22. The ceasefire took effect at midnight on Saturday Damascus time (22:00 GMT on Friday).
The UN Security Council unanimously adopted Resolution 2268 endorsing the Russia-US agreement on the cessation of hostilities in Syria on Friday, shortly before the ceasefire came into force.
The cessation of hostilities does not apply to designated terrorist organizations operating in Syria, including Islamic State (IS, also known as Daesh) and Jabhat al-Nusra (Nusra Front), a group affiliated with al-Qaeda.
