De Mistura abruptly aborted a first round of talks on Feb. 3, saying more preparatory work was needed from the "International Syria Support Group" (ISSG), led by the United States and Russia.
De Mistura said he would brief the UN Security Council Friday and then announce a date for a new round of talks, but he declined to comment further, Reuters reported.
"I will announce tomorrow when the parties (will reconvene) and the new talks (will happen)," he told reporters, as he arrived for a meeting of the ISSG task force on humanitarian access.
Another ISSG task force, on ceasefires, will convene for the first time in Geneva Friday, De Mistura said - an opportunity for diplomats from ISSG countries such as Saudi Arabia, Turkey and Iran to say whether they endorse the US-Russian proposal.
Combatants are required to say whether they will agree to the "cessation of hostilities" in the five-year war by noon on Friday (1000 GMT), and to halt fighting Saturday.