US and Russian military officials were coming to Geneva to discuss details, he told reporters after a weekly meeting of the humanitarian task force.
"We are all awaiting and urging the two co-chairs - which means Russia and the US- to expedite their own discussions on how to reduce violence," de Mistura said.
"I understand that there are several experts from the military establishment from both Russia and perhaps from the US on their way to Geneva and probably, most likely, in order to discuss the so-called 'devil in the details' which are the ones we have been asking to be sorted out as soon as possible."
US Secretary of State John Kerry and Russian Foreign Minister Sergei Lavrov have met in Moscow and Laos in recent weeks to try to hammer out a deal on military cooperation that is widely seen as a key to reinstating a nationwide ceasefire.
