(Adds airlines did not respond to request for comment, paragraph 5)

Feb 18 (Reuters) - The U.S. Justice Department has asked United Continental Holdings Inc for documents and oral testimony related to its contract to carry mail for the U.S. Postal Service, the airline said in its annual regulatory filing on Thursday.

The U.S. agency's investigation regarded "delivery scan and other data purportedly required for payment for the carriage of mail," the filing said. United, which received the civil inquiry last Oct. 13, said it is responding to the Justice Department (DOJ) but cannot predict what action, if any, the regulator might take as a result of the investigation.

United spokeswoman Megan McCarthy added in a statement that the civil demand addressed international mail scanning requirements of its U.S. Postal Service contracts.

"We believe the DOJ inquiry is industry-wide," McCarthy said. "We will continue to work with DOJ on its requests for information, and we are reviewing our mail scanning practices to ensure compliance."

American Airlines Group Inc, Delta Air Lines Inc , Southwest Airlines Co and JetBlue Airways Corp did not immediately respond to requests for comment.

Separately, United said in the filing that it is cooperating in a previously undisclosed investigation by the U.S. Securities and Exchange Commission, which is related to another government probe surrounding the airline and the Port Authority of New York and New Jersey.

United's Chief Executive Jeff Smisek resigned in September in connection with an investigation that has addressed whether the company added flights to Columbia, South Carolina to curry favor with then-Port Authority Chairman David Samson, who had a home there.

(Reporting by Jeffrey Dastin in New York; Editing by Matthew Lewis, Alan Crosby and Bernard Orr)