Many of the victims had gathered to collect fuel after the vehicle overturned close to railway tracks as it tried to park.
Another 37 people were injured in the explosion late on Sunday evening.
President Mahamadou Issoufou has traveled to visit some of the injured in hospital.
The latest accident happened on the RN1 road that links the Diori Hamani international airport to Niamey.
One eyewitness told Agence France-Presse: "Just before midnight I went out and I saw the truck overturned. People came from everywhere to take gas, then I saw fire on its side and everything burst into flames."