Syrian rebel groups wrote to the United States on Sunday, saying they would "cooperate positively" with a ceasefire but had deep concerns about details of the deal as relayed to them, two rebel officials said, Reuters reported.
The ceasefire was agreed between Russia and the United States, which backs some rebel groups, to come into effect at sundown on Monday, and excludes some jihadist groups.
"We in the revolutionary factions reconfirm our fixed position of dealing positively with the idea of a ceasefire," said the text of the letter as confirmed by the rebel officials.
But underlining the depth of rebel concern at the details, it cited reservations "linked to our survival and continuation as a revolution".