International monitors say they have found a new type of Russian-made rocket system in rebel-held eastern Ukraine.
The Organisation for Security and Co-operation in Europe (OSCE) spotted the powerful TOS-1 Buratino multiple rocket launcher in Luhansk.
Meanwhile, the leaders of Ukraine, Russia, Germany and France discussed peace efforts in Ukraine, with Paris and Kiev saying the pullout of light weapons would start on Saturday.
Moscow denies arming the rebels.
It also rejects accusations by Ukraine and the West that it is sending heavy weapons to the pro-Russian separatists in the Donetsk and Luhansk regions.
However, the Kremlin admits that Russian "volunteers" are fighting alongside the rebels.
The OSCE, which is monitoring the ceasefire in eastern Ukraine, said in a statement that it found the Buratino on a military training ground run by the so-called People's Republic of Luhansk 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.
A spokesman for the OSCE Special Monitoring Mission (SMM) to Ukraine told the BBC the discovery was particularly significant because of the damage the rockets could cause.
"This is a very destructive weapon which is fired indiscriminately," he said.
Russian-made Grad rockets have been used by both sides in the conflict, but the Buratino is a more powerful system.
TOS-1 Buratino multiple rocket system
- 24 rockets fired from mobile T-72 tank chassis
- Target area can be up to 40sq km (15.4 sq miles) if all 24 are fired
- 220mm warheads are incendiary (flame-thrower) or thermobaric (using oxygen at target area to strengthen blast)
- Range is from 2.7km (1.7 miles) to 3.5km (2.2 miles), depending on variant
- Crew targets rockets with laser and ballistic guidance instruments
- Soviet forces used Buratino in Afghanistan war in 1980s
- Also used by Russian troops in Chechnya
Sources: Russian Defence Ministry, TV Zvezda
Ukraine's military previously said the Buratino - nicknamed "scorched earth" in Russia - had been used against government troops in rebel-held eastern Ukraine.
But the OSCE told the BBC the sighting on 25 September was the first it had on record.
Only Russia produces the system, according to defence analysis group IHS Jane's and the Stockholm International Peace Research Institute.
Russian troops used it in Afghanistan in the 1980s against the Mujahideen and in Chechnya against separatist rebels in 1999-2000.
Russia has sold a more advanced version - Solntsepyok ("heat of the sun") - to Kazakhstan, Azerbaijan and Iraq.
However, experts say the Buratino was not exported to Ukraine before the conflict in the east of the country broke out last year.
In Paris, Russian President Vladimir Putin met Ukraine's President Petro Poroshenko, French President Francois Hollande and German Chancellor Angela Merkel on Friday for the first time since they agreed a peace deal for Ukraine in Minsk in February.
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.
Mr Hollande said the pullout would start on Friday, with Mr Poroshenko later confirming this.
The government in Kiev and the pro-Russian rebels earlier finally agreed to withdraw weapons of less than 100mm calibre from the front line.
Mr Poroshenko said this process would then continue in stages and should be completed within 41 days.
The French leader also said the elections in the rebel areas must be held according to Ukrainian law, as envisaged in the Minsk peace deal.
This point was reiterated by Mrs Merkel, who also noticed "progress" during the talks.
Mr Poroshenko said that the four leaders had supported the idea of the elections based exclusively on Ukrainian legislation and in the presence of OSCE observers, who should be granted full access.
However, the rebels said before the Paris meeting that they still intended to proceed with staging local elections on their terms.
Kiev says such elections - to be held on different dates from the rest of Ukraine and not according to Ukrainian law - would be "fake".
President Putin's office did not provide details of the Paris talks, saying only in a brief statement that the participants "synchronised watches" on the implementation of the key points of the Minsk deal.