Codes to gain access to United Airlines cockpits may have been made public, the carrier said on Monday, but it stopped short of confirming a report that a flight attendant inadvertently published the codes online in a potential threat to air security.

The airline still could keep its flight decks secure through other measures, Maddie King, a spokeswoman for United Continental Holdings Corp, said in an email. She declined to specify the other safeguards because of security considerations.

"We are working to resolve this issue as soon as possible," she said.

The Associated Press reported that King said the airline was working to change the codes on all of its aircraft.

Citing a pilot who was briefed on the matter, the Wall Street Journal reported on Sunday that United, the world's third-largest airline by revenue, had alerted pilots that access codes to unlock cockpit doors were mistakenly posted on a public website by a flight attendant.

Cockpit security emerged as a top priority for airlines in September 2001, when hijackers took control of United and American Airlines planes and crashed them into New York's World Trade Center and the Pentagon in Washington. A fourth airliner commandeered by jihadists crashed in a western Pennsylvania field.

The United unit of the Air Line Pilots Association said in a statement that the accidental leak of information showed the need for stronger protections for flight deck doors.

The union has long backed secondary barriers, which it said would cost $5,000 US each, and called on Congress to mandate them.

"The installation of secondary barriers on all passenger aircraft is a simple and cost effective way to bolster the last line of flight deck defense," the union said.