Hikes in the price of natural gas for vehicles (NGV) will be taken into account in working out how big the increase should be, and he would make a decision before Tuesday, he said.
ACM Prajin also appealed to private bus operators not to suspend services in the interim, as passengers would suffer.
Meanwhile, the Private Bus Operators Association on Tuesday submitted a letter to ACM Prajin via the ministry permanent secretary, calling for a fare rise of between eight baht and 11 baht for non-air-conditioned buses and three baht more for air-conditioned buses.
Association president Phattharawadi Klomcharoon said their demands must be met within a week, or the operators of 2,500 private buses may have to gradually suspend services.
They will not have enough money to pay for gas, she said.
Ms Phattharawadi also warned the central land transport committee's proposed one-baht fare increase would not be enough.
"This price [rise] does not reflect the real cost as it does not take into account increases in the price of NGV," she said.
Meanwhile, the network of taxi cooperatives in Bangkok yesterday demanded a 13% second-phase fare hike and a freeze on NGV prices.
"If the government fails to approve a 13% second-phase rise or if no other subsidy measures are rolled out, the same problems of us rejecting customers will occur," network chairman Witoon Naewpanich said.
The Transport Ministry has approved a 5% second-phase taxi fare rise, which will take effect in April.
An 8% first-phase rise took effect in December.