Zakharchenko said last week that separatist forces had been bolstered by "1,200 personnel who had trained for four months on the territory of the Russian Federation."
He also said "reserves have been amassed" of 150 armored vehicles, including 30 tanks, but did not specifically state that they came from Russia, according to a video of his speech posted online.
Following Zakharchenko's comments, the German government urged Moscow to "finally stop" deliveries of weapons to Ukrainian separatists, broadcaster Deutsche Welle reported, citing a government spokesman.
German Chancellor Angela Merkel also discussed the alleged supplies of Russian tanks to separatists during a telephone call with Ukrainian President Petro Poroshenko, the report said.
"The subject of the discussion was, above all, the reports confirmed by the separatists about the delivery of tanks from Russia and the reinforcement by fighters trained on Russian soil," Merkel's spokesman Steffen Seibert was quoted as saying on Saturday. "They agreed that these deliveries of weapons must finally stop and that a ceasefire must be agreed."