The state bank said it would wait for THAI's rehabilitation plan approved by the "Superboard" which oversees state enterprises' policies before making the decision.
THAI booked losses of 12 billion baht in 2013, which will take several years to clear.
However, a source at the ministry said the GSB had little choice but to lend the amount to THAI in September because THAI needed it.
If the ministry does not guarantee the loan, it will have to ask the Public Debt Management Office (PDMO) to repay GSB by early 2015, the source said.
"The lending will more likely be an emergency credit line or a short-term loan of four months starting in September. The ministry through the PDMO will then borrow to repay the GSB," the source said.
GSB board chairman Somchai Sajjapongse said the GSB's board had never considered lending to THAI and the next board meeting was set on Sept 18.
"If this is the case, we'll need to consider the purpose of the loan, as well as THAI's repayment capability and rehabilitation plan to minimise our risk. Although we have ample liquidity at present, any damage will be taxpayers' burden," he said.
Earlier, finance permanent secretary Rangsan Srivorasart, who is also a THAI director, said THAI might need 20 billion baht to pay for aircraft and ease its liquidity.
The ministry planned to have THAI borrow 5-7 billion baht from financial institutions in 2014.
For 2015, the ministry is considering whether to borrow under its name and let THAI borrow from it or have THAI borrow on its own with the ministry's guarantee.
THAI shares closed down 0.65% to 15.20 baht on Friday in trade worth 57.88 billion baht.