"Eleven people are at the hospital, including two children and an elderly woman. Three people are in a serious condition," news agencies cited health ministry spokesman Oleg Salagai as saying.
Regional governor Andrey Bocharov told Interfax news agency: "At this time, the fate of eight people is unknown."
The emergency situations ministry said operations were underway to determine "where they are at the moment", adding that the eight had lived in three of the apartments hit by the blast.
The blast sparked a violent fire that was finally put out late afternoon, said Oleg Grebenyuk, an official at the local emergency situations office.
"It is possible that some people are trapped under the rubble," Dmitry Ulanov, spokesman for the regional branch of the emergency situations ministry, told AFP.
Russian news agencies, citing sources among the rescuers, said four people may have been trapped under the rubble.
The head of the Volgograd district where the blast occurred, Sergei Tatsy, told state television that 36 apartments had been destroyed in total, leaving some 100 people homeless.
Russia's investigation committee, which has launched an inquiry, said the two injured children were aged five and seven and their lives were not in danger.
Gas explosions are common in Russia. The last serious accident, in February 2012, killed 10 people in the southern city of Astrakhan, destroying the entire side of a nine-storey building.
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