(RETUERS) - An unknown number of United Airlines customers purchased US flight tickets for practically nothing for a short period on Thursday when the company's website displayed some fares at zero but there's no guarantee anyone will be able to use the tickets.

"One of our filings today contained an error which resulted in certain fares displaying as zero," United Airlines spokeswoman Jennifer Dohm said in an email.

"We have corrected this error."

The airline said it temporarily shut down its website and stopped booking flights through its phone center for approximately two hours before resuming operations after the error was fixed.

Miscellaneous airport fees may have resulted in some passengers paying a nominal cost for the tickets. United was still debating if it will honour the fares purchased or if customers are out of luck.

"We are in the process of evaluating this and as always we will do what is appropriate," Ms Dohm said.