Alexander Zakharchenko, prime minister of the self-proclaimed Donetsk People's Republic, said the rebels were in the process of receiving some 150 armoured vehicles, including 30 tanks, and 1,200 fighters who he said had spent four months training in Russia.
"They are joining at the most crucial moment," he said in a video recorded on Friday. He did not specify where the vehicles would come from. Moscow has come under heavy Western sanctions over its annexation of Ukraine's Crimea and accusations it is supporting separatists in east Ukraine with fighters, arms and funds. Russia denies those charges. In a sign of concern at the latest rebel comments, German Chancellor Angela Merkel and Ukrainian President Petro Poroshenko agreed in a phone call on Saturday that deliveries of weapons to separatists in Ukraine must stop and a ceasefire must be achieved, a German government spokesman said.
The risk of outright war between the two most powerful former Soviet states was highlighted on Friday when Ukraine said it partially destroyed an armoured column that had crossed the border from Russia. The report triggered a sell-off in global shares.
But Moscow made no threat of retaliation, instead saying it was a "fantasy" that its armoured vehicles had entered its neighbour's territory.
U.S. Vice President Joe Biden also spoke to Poroshenko on Saturday, and the White House said: "The two leaders agreed that Russia's sending military columns across the border into Ukraine and its continued provision of advanced weapons to the separatists was inconsistent with any desire to improve the humanitarian situation in eastern Ukraine."
Ukrainian Foreign Minister Pavlo Klimkin called on NATO to provide military support for Ukrainian troops.
The rebels, who have ceded ground to government forces in recent weeks, have been promising a counter-offensive for several days but have yet to launch one.
Ukrainian native Zakharchenko took over from Russian citizen Alexander Borodai last week and his combative comments will probably dash hopes that changes at the top of the rebel leadership might signal willingness to end hostilities.