The Russian-US brokered ceasefire is due to start at sundown, but it has yet to win support from Syria's political or military opposition.
"We want to know what the guarantees are," said Salem al-Muslet, spokesman for the High Negotiations Committee, which groups political and military opposition factions.
"What is the definition that has been chosen for 'terrorism', and what will the response be in case of violations?
"We are asking for guarantees especially from the United States, which is a party to the agreement," Muslet told AFP.
In the text seen by AFP, rebels wrote they would "deal positively with the idea of the ceasefire," but did not explicitly back it.
"The clauses of the agreement that have been shared with us do not include any clear guarantees or monitoring mechanisms... Or repercussions if there are truce violations," they said.
Ahmad al-Saoud, who heads the US-backed Division 13 rebel group which signed the letter, said they had received no response to their concerns.
Questions remain, however, about how the ceasefire will apply in several parts of the country where the former Al-Qaeda affiliate Fateh al-Sham Front, previously known as Al-Nusra Front, is present.
The deal says that Washington and Moscow will begin joint targeting of jihadists including Fateh al-Sham in a week, if the ceasefire holds.
But the jihadist group is a powerful partner for many opposition factions on the ground, and the rebel letter to the US warned of repercussions if the group was targeted.
And Ahrar al-Sham, an ally of Fateh al-Sham in Idlib and other provinces, said late yesterday that the truce deal would only serve to boost the regime.
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