A United Airlines flight was diverted to Dallas on Monday out of "an abundance of caution" on the September 11 anniversary after two unclaimed objects were found aboard the plane, US security officials said.

Transportation Security Administration spokeswoman Yolanda Clark said United Flight 351 from Atlanta to San Francisco was diverted for security reasons.

"After the flight was in the air, an unclaimed Blackberry was found on the aircraft. The aircraft is secure, but the flight was diverted out of an abundance of caution," she said in Washington.

Clark said that before the plane took off, an unclaimed backpack was found on board and it was removed.

Local law enforcement and TSA will meet the aircraft when it lands.

Monday was the fifth anniversary of the 2001 hijacked plane attacks that killed nearly 3,000 people in New York, Washington and Pennsylvania.