The provincial government said six people were seriously injured while another 33 suffered slight injuries.
They were all being treated at the county hospital. The government identified the incident as a case of “arson” in a press release issued later, official Xinhua news agency reported.
The blast went off at the Rural Credit Cooperatives in Tianzhu Tibetan Autonomous County in the city of Wuwei at when all the bank employees were having a meeting.
A witness who declined to be named said someone ignited a homemade “gasoline bomb”.
He said, more than 10 people throwing themselves out of the window of the fourth-floor meeting room, while many others, with charred bodies and limbs, were carried out of the building on stretchers,
Police cordoned off the street for investigation shortly after the incident.
Tianzhu County is 128 km from Lanzhou, the provincial capital.
A third of the county’s 210,000 residents are Tibetans. The incident in the backdrop of tensions at Aba County of the southwest China’s Sichuan Province, which last month witnessed unrest between police and Tibetan protestors.
Earlier a 16-year-old monk self immolated on March 16 at Kirti Monastery in the Sichuan province.