MOSCOW — A shaky cease-fire in eastern Ukraine looked ever more tenuous on Sunday as European monitors confirmed reports of unmarked military vehicles driving through rebel-held territory while Donetsk, the region’s biggest city, endured a nightlong artillery battle.

The monitoring group, the Organization for Security and Cooperation in Europe, said that long columns of unmarked military vehicles, some towing howitzers, were spotted over the weekend. The monitors did not speculate as to the origins of the trucks or the people inside them, but Ukrainian officials said the statements bolstered their claims that Russia was again arming and training separatists.

The O.S.C.E. reported that its observers had driven on Saturday past a column of more than 40 trucks on a highway outside Donetsk. The trucks were covered with tarpaulins and “without markings or number plates — each towing a 122 mm howitzer and containing personnel in dark green uniforms without insignia,” the O.S.C.E. statement said.

On Sunday, after what journalists in Donetsk described as the heaviest night of artillery shelling in and around the city in at least a month, the O.S.C.E. observers saw two more unmarked military columns. The observers noted 17 trucks in each column, some equipped with Grad ground-to-ground rocket launchers and others towing more howitzers.