Three months will be needed between the vote of the electoral law and the election date, which will be overseen by the Organization for Security and Co-operation in Europe (OSCE), according to the French president.
The Russian defense ministry did not reply to written questions from Reuters about whether Ukrainian rebels were supplied with the weapon or where it had been exported.
As Russian Federation launched airstrikes in Syria to target rebels, the world thought Moscow has shifted its focus away from Ukraine.
Alexander Hug, deputy chief monitor of the OSCE monitoring mission to Ukraine, told Reuters by phone monitors had spotted the Buratino at a rebel training area in the village of Kruhlyk.
The rockets have two types of warhead – either incendiary, which can spread flames over tens of kilometres, or thermobaric, which sucks up oxygen to boost the explosion.
Russian-made Grad rockets have been used by both sides in the conflict, but the Buratino is a more powerful system.
Ukraine and pro-Russian separatists agreed earlier this week to extend a pull-back of weapons in east Ukraine to include tanks and smaller weapons systems.
But the OSCE told the BBC the sighting on 25 September was the first it had on record.
Only Russian Federation produces the system and it was not exported to Ukraine before the conflict broke out, according to IHS Jane’s Group and the Stockholm global Peace Research Institute, which track arms exports.
Russian President Vladimir Putin is meeting the leaders of Ukraine, France and Germany on Friday in a revived European push to bring peace to eastern Ukraine.
The so-called Normandy Four meeting assessed all elements of the deal, including the staging of local elections in the rebel-held regions and the withdrawal of heavy weapons from the line of contact.
Ukrainian President Petro Poroshenko said he felt “cautious optimism” on the results of Friday’s talks, Interfax Ukraine quoted him as saying. If that continues, he said, “then I will argue for lifting the sanctions“. It was not immediately clear how the developments could impact the outcome of a long awaited peace summit in Paris where Ukraine’s president announced the start of withdrawal of heavy weapons and leaders spoke of postponing local elections in Ukraine to ease tensions.