Authorities lost contact with the helicopter on Wednesday as it carried Governor Igor Farkhutdinov and other officials and businessmen from Petropavlovsk-Kamchatsky in Kamchatka to the Kuril Islands in the Pacific Ocean.
The Kurils are part of the Sakhalin region.
The search failed to yield results before it was suspended at nightfall, said Emergency Situations Ministry spokeswoman Irina Andriyanova.
A total of 346 rescuers from the Emergency Situations Ministry and the Defense Ministry were taking part in the search over an area of approximately 400 square kilometers, the Emergency Situations Ministry said.
Some 41 ships and 19 aircraft were involved, it said.
Confusion lingered as to exactly how many people were on board the Mi-8 helicopter. Officials first said there were 17 people, but later the ministry said there were 20 -- 17 passengers and three crewmembers.
President Vladimir Putin's representative in the Far East, Konstantin Pulikovsky, said the absence of a list of passengers is a serious violation of flight regulations, Itar-Tass reported.
Pulikovsky also said Khalaktyrskiye Airlines, which owns the helicopter, is not accredited to transport high-ranking officials.
Federal prosecutors in the Far East opened a criminal investigation into possible violations of flight regulations, Interfax said.
The missing helicopter had been equipped with a satellite telephone, signal flares and a radio-beacon, said Pulikovsky's deputy, Alexander Drozdov.
"We cannot rule out a collision with a volcano, but we are still hoping for the best," Drozdov was quoted by Interfax as saying.
Pulikovsky rejected the possibility that a helicopter could have been shot down by a rocket during military exercises that opened in the region on Monday.
In televised remarks, Pulikovsky said the active phase of the exercises would be held only next Wednesday.
Helicopters are a key form of transport in many hard-to-access parts of the Arctic and Far East, but the country's helicopter fleet is aging and crashes are common.
Krasnoyarsk Governor Alexander Lebed was killed in a crash when his helicopter became tangled in power lines in April 2002.
Two pilots are on trial in Krasnoyarsk for violating flight rules in the crash that killed Lebed.
Defendant Takhir Akhmerov testified Thursday that the helicopter was flying low at the insistence of Lebed, who wanted a view of the highway below, Itar-Tass reported.
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