The opposing sides in eastern Ukraine – government troops and pro-Russian separatists, are both withdrawing tanks and other weapons from the front line, Ukraine’s military command says.
The pullback is part of the ceasefire accord signed in Minsk in February.
A spokesman for international monitors in the area, Michael Bociurkiw of the OSCE, said there was “encouraging” movement of heavy weapons, but storage sites would still have to be verified.
The move is happening first in Luhansk.
Later the forces in Donetsk region are also scheduled to withdraw weapons from the front line.
In both regions the process is supposed to be completed within 41 days. On either side of the line the weapons, measuring up to 100mm (4in) calibre, are to be pulled back a distance of 15km (nine miles).
A statement from the rebels in Luhansk said Ukrainian government weapons were being withdrawn yesterday. The rebels said their own withdrawal had also begun.
Since February the Organization for Security and Co-operation in Europe (OSCE) has reported numerous violations of the Minsk ceasefire, including skirmishes with heavy armour and artillery.
But for more than a week, there have been no major violations.
On Monday the OSCE’s Michael Bociurkiw told the BBC by phone that “we recorded movement of heavy weaponry; it will take some time to verify that the weapons are stored”.
“There are some encouraging developments, it remains to be seen if they are taken to storage areas and put out of harm’s way,” he said.
The OSCE will need the co-ordinates of the storage sites, as well as inventory lists with details of weapons types and serial numbers, he explained.
A Ukrainian military spokesman, Ruslan Tkachuk, said the “synchronised withdrawal from the front line” began at 11:00 local time (08:00 GMT) in Luhansk region – involving T-64 and T-72 tanks, as well as anti-tank cannon and mortars.
The pullout date was announced at a meeting of leaders from Ukraine, Russia, France and Germany on Friday.
Moscow denies sending troops and heavy weapons to the separatists, who hold a large swathe of the Donetsk and Luhansk regions.
However, the Kremlin admits that Russian “volunteers” have been fighting alongside the rebels.
The insurgency in the east erupted in April 2014 after Russia’s annexation of Crimea.