The blast, the third in Russia in the past six weeks, occurred Sunday evening in the town of Nevinnomyssk after most of the passengers had left the bus, which was making a four-hour trip from Pyatigorsk to Stavropol.
Rescuers said two women died in the explosion, but they had not yet been identified as their bodies were badly burnt. "The bus burnt to the ground," an emergencies service spokesman said.
Five people are still in hospital, a local hospital official said.
Forensic examination has shown that the explosive device was stuffed with metal balls, and had the power of 500 grams of TNT, a law enforcement official in the southern federal district said.
The official also said it was suspected that the blast had been organized by ring leaders from Russia's troubled North Caucasus. "There are reasons to believe that this terrorist attack had been masterminded and carried out by leaders of militants operating in the North Caucasus," the official said.
Russia's republics in the North Caucasus still suffer from a spillover of violence in the Chechen republic, which remains disrupted by sporadic bombings and clashes between gunmen and federal troops, even though the active phase of the North Caucasus antiterrorism campaign officially ended in 2001.
On November 22, five people were killed, including a child, and 13 wounded by an explosion on a bus going from Pyatigorsk to Vladikavkaz in the North Caucasus.
The law enforcement official said the explosion on November 22 and Sunday's bus blast had been organized by the same people. He added that in mid-October, police in the southern federal district had warned that a militant ring leader, Doku Umarov, was preparing a series of terrorist attacks across Russia.
In late November, Ivan Sydoruk, a deputy prosecutor for the southern federal district, said terrorists had obtained over $1.5 million to destabilize the situation in the North Caucasus.
In another terrorist attack, a bus was blown up in Togliatti, the center of European Russia, on October 31. Eight people were killed and 70 injured, most of them students.
