The Organisation for Security and Co-operation in Europe (OSCE) spotted the powerful TOS-1 Buratino multiple rocket launcher in Luhansk.
In airstrikes this week, the Russian military has bombed areas outside Islamic State strongholds, including one primarily held by Western-backed rebels, USA officials say.
Rebel leaders this week signed an agreement to extend a withdrawal of weapons to include tanks and smaller weapons systems.
However, the Kremlin admits that Russian “volunteers” are fighting alongside the rebels.
The humanitarian situation in the parts of Donbas not controlled by Kyiv remains complicated, but people believe that there is still a chance of normalizing living conditions in the region, Deputy Chief Monitor of the OSCE Special Monitoring Mission (SMM) to Ukraine Alexander Hug has said.
“This is a very destructive weapon which is fired indiscriminately,” he said.
But the OSCE told the BBC the sighting on 25 September was the first it had on record.
Russian troops used it in Afghanistan in the 1980s against the Mujahideen and in Chechnya against separatist rebels in 1999-2000.
Enormous inflation rates have also been observed in the territories not controlled by the Ukrainian government, he said.
“We saw the weapon on that training ground”, Hug said.
Putin will be in Paris for a peace summit on the Ukraine conflict, but Russia’s sudden intervention in Syria looks set to dominate as he holds talks with French President Francois Hollande and German Chancellor Angela Merkel. The withdrawal is expected to take 39 days.
But Putin has slowly shifted Western attention away from the conflict by boosting Russia’s military presence in its war-torn ally Syria – a step seen by Washington as an effort to boost the standing to President Bashar al-Assad.
Jean Asselborn, the foreign minister of Luxembourg, which now holds the rotating presidency of the European Union, said the message had already been passed clearly to Moscow: If the rebels go ahead with their own elections, it would be a huge blow to the Minsk peace process-and could revive discussions within the bloc about tighter sanctions on Russian Federation.