Mir, which was flooded in August 2017 and has been idle since, will be subject to deep-level exploration down to about 1,300m to confirm the mine's reserves. That step is not projected to wrap up until early 2022.

"Based on the results of the exploration works, pilot holes will be drilled to start preparation of deposit opening design documents within one to two years," the company said. "This work is to be completed before 2024."

At the same time, Alrosa said it will be examining conceptual design activities for water disposal at the mine and will select ventilation and gas safety technology that will work the best with Mir.

If it is decided that restoration of the underground operation is feasible, the miner said construction could take six to eight years.

"Mir mine restoration can start no earlier than 2024 and only if the studies yield positive results, and if it is confirmed that construction and mining can be done with the highest level of occupational safety," officials stressed.

"Alrosa's decision about further development of this mine will be based solely on safety considerations and economic viability."

Alrosa, which said that it has the largest rough diamonds reserves in the world, accounts for about one-third of the world's rough diamond production.