The Paris-based opposition Syrian National Council rejected the Geneva deal as too ambiguous, according to the AP news agency.
Violence has worsened in Syria recently despite the cease-fire mediated by Mr Annan as part of his six-point plan earlier this year.
On Friday, government forces recaptured the Damascus suburb of Douma - an opposition stronghold - after 10 days of artillery bombardment. Activists described conditions in the town as "catastrophic".
Activists estimate that as many as 15,800 have died since the uprising began early last year. Casualty figures are difficult to verify, as Syria does not allow foreign journalists to operate on its territory.
The conflict is seen as becoming increasingly militarised, with both rebels and government forces thought to be receiving arms supplies from abroad.