Pro-Russian separatist leader Alexander Zakharchenko has announced plans to recruit 100,000 men, as fighting with Ukrainian forces intensifies.
The separatists are trying to capture the key town of Debaltseve, while central areas of rebel-held Donetsk have been hit by artillery fire.
Dozens of civilians were killed on both sides of the front line at the weekend.
"Mobilisation will start in 11 days' time," Mr Zakharchenko told Donetsk news agency.
Hundreds of civilians have been moved out of Debaltseve amid reports that the separatists have reached the outskirts of the town.
A fragile ceasefire agreed in Minsk in September collapsed in eastern Ukraine last month as rebels stepped up their offensive on several Ukrainian positions and seized Donetsk airport. Thirty civilians were killed when shells were fired on the south-eastern city of Mariupol and international observers said the firing had come from rebel areas.
Ukrainian forces have been blamed for deadly artillery attacks on Donetsk city and other rebel-held areas.
Attempts to negotiate a new halt to violence failed on Saturday in the Belarusian capital Minsk when some of the rebel representatives did not turn up.
'Reserve force'
Mr Zakharchenko, head of the self-proclaimed Donetsk People's Republic, emphasised that recruits to separatist forces would be voluntary, Donetsk news agency said.
"It is a reserve force which will definitely be able to resist any attack," he was quoted as saying.
They would have a month's combat training with the aim of providing three motorised brigades for counter-attack, Interfax news agency reported..
Ukraine's government has also announced plans to boost its forces in the east. Tanks and other heavy military vehicles were seen heading to towards Debaltseve on Sunday in an attempt to bolster defences in the town.
The rebels' ability to raise large numbers of forces is unclear, although Ukraine has alleged they are being helped by as many as 9,000 Russian servicemen, a claim the government in Moscow denies.
A Canadian journalist who is in the conflict zone said the rebels had been involved in a tough battle as they tried to encircle Debaltseve.
"It's been quite surprising to hear how many have said they are experiencing significant losses and many of their fellow fighters have been killed. That's not something that you often hear," Kristina Jovanovski told the BBC World Service Newsday programme.
Military officials in Kiev said on Monday that five soldiers had been killed in the past 24 hours, although separatists said the number was more than 20. Thirteen civilians were killed in separatist-held areas during the same period, rebel sources said.
The latest fatalities bring to at least 60 the number killed during a weekend of heavy violence in the eastern regions of Donetsk and Luhansk.
