Pro-Russian separatist leader Alexander Zakharchenko has announced plans to recruit 100,000 men, as fighting with Ukrainian forces 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.
A fragile ceasefire agreed in early September in the Belarusian capital, Minsk, collapsed last month when rebels stepped up their offensive on several Ukrainian positions and seized Donetsk airport.
Rebels were accused of carrying out an artillery attack on the south-eastern Ukrainian port city of Mariupol which left 30 civilians dead.
Ukrainian government forces have been blamed for deadly attacks on Donetsk city and other rebel-held areas last month in which dozens of civilians were also killed.