Alrosa, Russia's largest diamond producer, said in a statement that "nine people are being searched for" after water broke through into the Mir mine in the Sakha region some 4,160 kilometres (2,600 miles) east of Moscow at around 0730 GMT.
The emergency situations ministry for the region said rescuers were "carrying out a search of the mine's workings in order to find and save people".
The water leaked into one of the mine's pumping stations out of a flooded disused crater that contained some 300,000 cubic metres of water, or the equivalent of 120 Olympic-size swimming pools, the emergency situations ministry said.
The accident is believed to have been caused by an "uncontrolled increase in the flow of water" out of the mine's abandoned crater into the underground shaft, Alrosa said.
It said this was caused by sudden geological processes and the washing away of rocks in the crater but insisted that all of the mine's "equipment has been regularly tested."
The Sakha region's branch of the Investigative Committee, which probes serious crimes, said in a statement that it was carrying out a check into possible safety violations at the mine.
Russian television said the rescue operation at the mine named Mir, or peace in Russian, was hindered by a power cut that stopped the lift system from working.
Channels showed footage posted on social media of water flooding into a control room and coming from the ceiling of underground passageways.
Alrosa said that 133 people have been evacuated from the mine. Two were taken to hospital with minor injuries, the region's health ministry said.
