Jubeir claimed on Sunday that Syria's government and its ally Russia had violated the truce, and that there would be an alternate plan if it became clear Damascus and its allies were not serious about the ceasefire. He did not give details on the plan.
"What Adel al-Jubeir says ... about a plan B regarding the current developments in Syria is merely a delusion in the mind of the Saudi regime," a Syrian foreign ministry official was quoted as saying in a statement on state media.
"Jubeir's statements are ... an attempt to thwart the cessation of combat operations," the statement added, Reuters reported.
Saudi Arabia supports militants fighting against Syrian President Bashar al-Assad, and backs the main Syrian opposition body.
The fragile cessation, drawn up by the United States and Russia, has led to a dramatic reduction of violence in Syria.
A Syrian military source has said the army is not violating the agreement.