President Vladimir Putin has vowed to "annihilate terrorists" following two deadly bomb attacks in less than 24 hours in the southern Russian city of Volgograd.
The remarks, made in Putin's New Year’s Eve address, came amid increasing security fears ahead of the Winter Olympics.
They were his first public comments since suicide bombers killed at least 34 people in attacks on a railway station and a trolleybus on Sunday 29 December and Monday 30 December.
The bombings raised fears that there will be more terrorist attacks in the lead-up to Russia hosting the Winter Olympics in less than six weeks in the Black Sea resort of Sochi.
"I am certain that we will fiercely and consistently continue the fight against terrorists until their complete annihilation," Putin said on a visit to the east coast city of Khabarovsk, according to Russian news agencies.
Police on Tuesday detained dozens of people in sweeps through Volgograd, although there was no indication that any of them was connected to the attacks, for which nobody has claimed responsibility.
Putin has staked his reputation on the Winter Games in Sochi, which lies at the western edge of the Caucasus Mountains.
But he has been unable to crush Islamist militants in the Muslim provinces of the North Caucasus, who want to leave Russia and form a separate Islamic State.
Putin ordered increased nationwide security after this week’s attacks, the deadliest outside the North Caucasus since a suicide bomber from a province next to Chechnya killed 37 people at a Moscow airport in January 2011.
In Volgograd, more than 5,000 police and interior troops were mobilised in "Operation Anti-terror Whirlwind," a spokesperson for the Interior Ministry said on state TV.
Andrei Pilipchuk said that 87 people had been detained after they resisted police or could not produce proper ID or registration documents, and that some had weapons. But there was no sign any were linked to the bombings or suspected of planning further attacks.
The Itar-Tass news agency said police were focusing on migrant workers from the Caucasus and ex-Soviet states - groups that rights activists say face prejudice and are often targeted by police indiscriminately.
Investigators said they believed a male suicide bomber was responsible for Monday's morning rush-hour blast, which turned a trolleybus into a twisted wreck and left bodies lying in the street.
In Sunday's attack on the station, authorities initially described the bomber as a woman from Dagestan, but later said the perpetrator may have been a man.
The violence raises fears of an intensive campaign before the Olympics, an important project for Putin, who secured Russia's first post-Soviet Games in 2007, during his initial 2000-2008 stint as president.
Intended to showcase how Russia has changed since the collapse of Soviet communism in 1991, the Games have also been a focus for complaints in the West and among Russian liberals that Putin has stifled dissent and encouraged intolerance.
This month, Putin freed jailed opponents including oil tycoon Mikhail Khodorkovsky and the Pussy Riot punk band in what critics said was an effort to disarm Western criticism and improve his image.
In an online video posted in July, the Chechen leader of insurgents who want to carve an Islamic state out of mainly Muslim provinces south of Volgograd, urged militants to use "maximum force" to prevent the Games from going ahead.
International Olympic Committee President Thomas Bach condemned the "despicable attack on innocent people" and said he had written to Putin to express condolences and confidence that Russia would deliver "safe and secure games in Sochi."
The U.S. government said it was concerned Islamist militants may be preparing attacks aimed at disrupting the Olympics and has offered closer cooperation with Russia on security.
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