MOSCOW — Separatists in eastern Ukraine on Tuesday proclaimed a new state that aspires to include not only the areas they control but also the rest of the country. But Russia, their chief backer, sought to play down the announcement, saying it was merely part of public discussion.

The surprise announcement in the rebel stronghold of Donetsk casts further doubt on the 2015 cease-fire deal that was supposed to stop fighting in Ukraine’s industrial heartland and bring those areas back into Kiev’s fold while granting them wide autonomy. Some rebels said they have no intention of joining the new state.

More than 10,000 people have died in fighting since Russia-backed rebels took control of parts of Ukraine’s Donetsk and Luhansk regions in April 2014 after Russia annexed Ukraine’s Crimean peninsula. The rebels originally sought to join Russia but the Kremlin stopped short of annexing the area or publicizing its military support for the rebels.

Donetsk separatist leader Alexander Zakharchenko said in comments broadcast on Russian television that rebels in Donetsk and Luhansk as well as representatives of other Ukrainian regions would form a state called Malorossiya.

Most of the areas which are currently part of Ukraine were referred to as Malorossiya, or Little Russia, when they were part of the Russian Empire.