The explosive used in the attack appeared to be homemade, reports Russian video news outlet RT, and was packed with shrapnel. A 12-year-old girl was among those killed.
"About 15-20 minutes before the start of the concert we heard an explosion. We saw the blast had practically flung aside the crowd that had gathered outside ... about 100-150 people," Rustam, an eyewitness, told Ekho Moskvy radio, according to Reuters.
The city has rarely been the target of terror strikes. RT quoted Nikolay Petrov from the Carnegie Moscow Center as saying the attack signals spreading terror in the region. The city is 150 miles from Sochi, planned site of the 2014 Winter Olympics, where Putin has staked $17 billion of the state's money and his own personal prestige on the upcoming Olympic Games.
Many are worried that the unrest in the Caucasus could disrupt the Games.