The Russian authorities have opened a criminal case in terrorism, murder and the illegal use of firearms.

The statement was made by Vladimir Markin, of Russia's Investigative Committee, which is the equivalent of the US FBI, cited by The BBC and the Russian RIA Novosti news agency.

Markin spoke in the aftermath of an explosion that killed at least five people on a bus in the southern Russian city of Volgograd early Monday afternoon.

The blast happened just after 2:00 pm (10:00 GMT). It also injured more than a dozen people, some seriously.

The explosion was caused by a bomb, according to Russia's National Anti-Terrorism Committee.

Earlier reports had suggested the blast may have been caused by an exploding gas canister.

Investigators suspect a female suicide bomber, Russian news agencies report. Some have quoted sources at the Committee as saying the suspected suicide bomber was believed to be the partner of a militant Islamist.

There were about 40 people on board the bus, the BBC's correspondent in Moscow informs.

Bus services in Volgograd have been suspended as all vehicles have been ordered back to their depots to be searched for any sign of explosives.

Cities to the south, in Russia's North Caucasus, have seen attacks by Islamist militants in recent years.

The BBC further reminds that Russian President Vladimir Putin has stepped up security in the North Caucasus ahead of the 2014 Winter Olympics, which open in the Black Sea resort city of Sochi on February 7.

Separatists in Chechnya have fought two wars with Russian forces over the past two decades.

But the violence has spread across the North Caucasus in recent years, including to mainly-Muslim Ingushetia and Dagestan.

Hundreds of people, including members of the government and security services, have been killed.