There was no immmediate evidence of criminal intent, he said.
Some media reports said fireworks had gone off accidentally.
"It was a home-made device that exploded," the spokesman said.
Interfax news agency, quoting an emergency situations ministry official, said the blast was linked to fireworks stored inside a market stall that was selling underwear.
"Four women who were near the stall stuffered wounds and burns, according to initial reports," the official said.
Kabardino-Balkariaya blasts
Elsewhere in Russia on Saturday, five people, four of them police officers, were injured in explosions set off by anti-government fighters.
In a separate incident in the same province - Kabardino-Balkariaya - police tracked down a suspected fighter on Saturday.
He refused to surrender and was killed in a skirmish, the regional office of the interior ministry said.
The ministry's branch in Dagestan, in the northern Caucasus, said three police officers were injured on Friday when their vehicle hit a radio-controlled landmine laid by fighters on the outskirts of the provincial capital, Makhachkala.
In neighbouring Chechnya, a police officer and a civilian were wounded on Saturday by an explosive device left by fighters at a construction site, the regional branch of Russia's interior ministry said.
Large-scale battles in Chechnya ended years ago, but small groups of fighters stage regular attacks against authorities.