KIEV, Ukraine — In a long-awaited development, Ukrainian forces and separatist fighters both announced Thursday they are pulling back heavy weapons from the front line in eastern Ukraine.

Ukraine’s Defense Ministry said it reserved the right to revise its withdrawal plans in the event of an attack by rebel forces, however.

The pullback was supposed to have started over a week ago under a peace deal agreed upon earlier this month by the leaders of Russia and Ukraine to end the fighting in eastern Ukraine that has killed nearly 5,800 people since April. The intensity of fighting has declined notably in recent days, despite daily charges by both sides that the other is violating the Feb. 15 cease-fire.

Military spokesman Anatoly Stelmakh told the Interfax-Ukraine news agency that government forces on Thursday started moving 100-mm anti-tank guns back to the 25-kilometer (16-mile) minimum stipulated by the peace deal.

AP reporters on Thursday also saw rebel forces moving at least six 120-mm self-propelled howitzers from the front line near Olenivka, a town south of the rebel-held stronghold of Donetsk.

Monitors from the Organization for Security and Cooperation in Europe planned to report later on the progress of the withdrawal. The OSCE has hundreds of monitors in the region.

In Rome, NATO Secretary General Jens Stoltenberg said he welcomed indications of reduced fighting but renewed claims that Russia has supplied separatists with large quantities of weapons.

“Russia has transferred in recent months over 1,000 pieces of equipment — tanks, artillery, advanced air defense system — and they have to withdraw this equipment and they have to stop supporting the separatists,” Stoltenberg told reporters Thursday.

Russia denies charges that it arms and supports the rebels.

Michael Bociurkiw, a spokesman for the OSCE monitoring mission, said the weapons withdrawal required both sides to inventory their arms and provide details about how and where they are to be relocated.

“It’s not enough to be invited to follow the removal process part of the way. It has to be complete,” he said. “It’s not a shopping list, you cannot pick and choose.”

Ukraine’s military said Thursday its positions had not been shelled the previous night but military spokesman Col. Andriy Lysenko spoke of isolated armed confrontations, including ones near Donetsk.

The rebels claimed Tuesday to have begun their heavy weapons pullback, but that has not been independently confirmed.

Eduard Basurin, spokesman for the separatist forces, told the Russian TV station LifeNews that withdrawals from five locations were planned for Thursday, monitored by the OSCE. The locations he named included Olenivka, where AP journalists saw the 120 mm self-propelled howitzers being moved.

“The OSCE mission has been provided with all the documents they requested, which detail where equipment would be transported from and in which direction,” Basurin told LifeNews.

Kiev has until now demurred from pulling back its heavy weapons, insisting that the separatists fully observe the cease-fire. That stand was dismissed as “ridiculous” by Russian Foreign Minister Sergey Lavrov.

“Everyone understands that there isn’t an ideal truce and an ideal regime of ceasing fire,” Lavrov said Thursday.

Story compiled with information from AP reports.