Xinhua had said earlier that the blast occurred on the bus.
"Initial investigation, however, showed that the bus ... was blown up by the explosion of a truck," it said.
The bus was travelling north from Shenzhen, which borders Hong Kong, to the eastern province of Zhejiang.
The blast occurred early in the morning near the city of Shangrao in Jiangxi province.
At least seven people were wounded, but Xinhua said the exact death toll was difficult to determine because of the force of the explosion, which damaged nearby houses.
A police officer from Shangrao said police and doctors had been sent to the site to investigate the cause of the blast.
"We still don't know how many people were in the bus. But one thing is certain - nobody in the bus is alive," said the officer, who asked not to be identified.
Explosives or fireworks stored or transported illegally kill hundreds each year in rural China.
The truck was from Liuyang City in the central province of Hunan, a major firecracker producer, Xinhua said.
Twenty primary school children were killed earlier this month when dynamite stored illegally in a house exploded, causing their nearby classroom to collapse on them.