ARTEMIVSK, Ukraine — A fragile, European-brokered cease-fire deal for Ukraine appeared at risk of unraveling, at least in part, when a rebel leader announced Saturday night that he would not be bound by the accord on a central battlefield of the war.

Although the Ukrainian president and the pro-Russian rebel leader both announced that the cease-fire had gone into effect, the separatist leader said it did not apply to Debaltseve, where thousands of Ukrainian troops have been under siege and might be surrounded. The rebel leader, Aleksandr Zakharchenko, said the town, a critical railway hub, had not been mentioned specifically in the cease-fire agreement.

That deal, which went into effect at midnight, had been brokered by Chancellor Angela Merkel of Germany and the French president, François Hollande, in Minsk, Belarus. Neither leader immediately commented on Mr. Zakharchenko’s announcement about Debaltseve, which had been the scene of intense fighting in recent days as the rebels battled to take the town before any cease-fire.

Just after the midnight deadline, Ukraine’s president, Petro O. Poroshenko, went on national television saying he had ordered Ukraine’s armed forces to halt their fire in the contested eastern portion of the country. In his earlier statement, Mr. Zakharchenko, the head of the self-declared Donetsk People’s Republic, also said he had ordered his forces to halt combat all along the front line in eastern Ukraine, as required in the Minsk agreement, but not near Debaltseve. He also indicated that rebel forces would not allow the approximately 8,000 Ukrainian troops who are there to leave.