The sect members, including four children, went into the cave in the central Russian Penza Region in November to "save themselves during the time of the Apocalypse," which they say is due in May. They have threatened to set themselves on fire if any attempt is made to remove them from their underground shelter by force.
The group's founder, 43-year-old Pyotr Kuznetsov, earlier said that the sect had "spent one and a half months, often working nights," digging out and expanding the cave.
"The cave where the sect members are located is gradually collapsing due to flooding by thaw waters. If this goes on, there is a possibility that these people's lives could be threatened," the official said.
He said a round-the-clock rescue post had been established in the event of an emergency, and that local rescue workers were checking the condition of the cave every day.
Kuznetsov has been held in an asylum in Penza, about 600 km (370 miles) southeast of Moscow, since November. On Wednesday he was declared mentally unfit to stand trial. He had earlier been charged with inciting religious and racial hatred. He has recently explained his decision not to join his followers in the cave by speaking of "another mission in life."
The group's leader is now an 82-year-old woman known only as Antoniya.
The sect announced on Wednesday that they would come to the surface in late April or early May.
"It's possible that the sect members realized that their lives could be in danger if they remained underground. They have had to urgently strengthen the walls of the cave recently," said a spokesman for the local prosecutor, adding that large lumps of soil had also fallen into the sect's 'home'.
It is not clear how the promise to emerge from the cave ties in with the group's belief concerning May and the destruction of the planet.
