* Citizens' help needed to prevent "terrorist" attacks - top investigating body
* Suicide attacks have increased security concerns
* Winter Olympics open in Sochi on Feb. 7
By Gabriela Baczynska
MOSCOW, Jan 14 (Reuters) - Less than a month before the Winter Olympics in Sochi, Russian investigators appealed to civilians on Tuesday to be more vigilant and help avert the threat of "terrorist" attacks.
Insurgents who want to create an Islamist state in the North Caucasus mountains have threatened to attack the Games and suicide bombers killed at least 34 people in the southern city of Volgograd last month.
President Vladimir Putin, who has staked his personal and political prestige on next month's Games, has put troops on combat alert in Sochi and tightened security across the country but fears of an attack on the Olympics remains.
"You can't just count on the special services and heroic commandos," Vladimir Markin, spokesman for Russia's national Investigative Committee, which answers directly to Putin, told Izvestia newspaper in an interview.
"If our citizens are not ready to be vigilant, and to refrain from the shady business and corruption that create an environment for the terrorist underground, the intelligence services and - even more so - the investigators can only ... face the tragic consequences."
Markin did not specifically refer to the Olympics but that is currently the country's main security concern.
Markin said his agency had lost count of how many militants had been killed in Russia, which has fought two separatist wars in Chechnya in the North Caucasus since the Soviet Union's collapse in 1991.
Markin said federal security agencies cooperated well but civilians' help was still vital.
"In my view this is the only way, simultaneously from the top and bottom ... to control channels of illegal trafficking and terrorism," he told the paper, which is allied to the Kremlin.
Sochi lies on the Black Sea and on the western edge of the Caucasus Mountains, where the Islamist insurgents wage almost daily violence.
The insurgency's leader and Moscow's most-wanted man, Doku Umarov, has urged militants to exercise "maximum force" to prevent the Games going ahead.
Putin said after the Volgograd bombings that he would annihilate "terrorists" and Prime Minister Dmitry Medvedev has said he is confident Russia can guarantee security at the Games, which start on Feb. 7.
About 37,000 personnel are now in place to provide security in Sochi and the International Olympic Committee has expressed confidence the Games will be safe.
But, underlining the danger of attacks, security forces said on Saturday they had arrested five members of a banned militant group in southern Russia and defused a homemade bomb packed with shrapnel. (Editing by Timothy Heritage and Ralph Boulton)