Pro-Russian separatist leader Alexander Zakharchenko says rebels aim to boost their forces to 100,000, as fighting with Ukraine's military intensifies.
The rebels want to push government forces out of the eastern regions of Donetsk and Luhansk and are trying to capture the key town of Debaltseve.
Dozens of people including civilians were killed in clashes and artillery fire at the weekend.
Attempts to agree a truce failed when rebel negotiators did not turn up.
Denis Pushilin, a rebel representative, said rebels would only return to negotiations if the government declared a unilateral ceasefire.
A fragile ceasefire agreed in early September in the Belarusian capital, Minsk, collapsed last month when rebels stepped up their offensive in several areas and seized Donetsk airport.
Both sides have blamed each other for dozens of recent civilian deaths resulting from the fighting.
Mr Zakharchenko, the head of the self-proclaimed Donetsk People's Republic, said mobilisation was "urgent" and that the call-up would take place in the next 10 days.
"This is to increase our army to 100,000 people," he said. "It does not mean we will take in 100,000, but the joint army of Donetsk and Luhansk People's Republics' army should be 100,000."
However, the rebels' ability to raise large numbers of forces is unclear.
'Not prepared for truce'
The latest attempt at a negotiated ceasefire collapsed on Saturday, with the Organization for Security and Cooperation in Europe (OSCE), which is involved in the talks along with Russia, saying the rebels "were not even prepared to discuss implementation of a ceasefire and withdrawal of heavy weapons".
The 28-nation European Union last week extended through September a first wave of targeted sanctions it had slapped on Moscow and Crimean leaders in the wake of Russia's March seizure of the Black Sea peninsula from Ukraine.
But deep divisions within the EU meant that there was no agreement on expanding broad sanctions targeting Russia's economy.
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