She said on Monday that the Commerce Ministry has been in charge of the rice deal.
Former finance minister Korn Chatikavanij said in his posting on Facebook that the deal may not exist, referring to a Reuters report.
On his return from China earlier this month, Niwatthumrong said that China agreed to place orders for 1.2 million tonnes of Thailand's white rice, along with orders for tapioca and rubber.
Reuters however said in an article dated September 12 that industry officials were unsure why China would buy so much rice at a time when demand was weak and there were abundant cheaper supplies elsewhere, while a Chinese source said the two sides were still in talks.
An industry source close to the State Grain Administration, China's state body responsible for grain trading and distribution, said talks on a government-to-government deal had been taking place with Thailand for some time but nothing had been settled.
"It is impossible for China to import that much rice as China doesn't need that much," the source said, referring to the 1.2 million tonnes.