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.