The threat was described as "current" but not imminent, said the official.
"They have trained women," said former White House counterterrorism official Richard Clarke, an ABC News consultant.
Separately, Britain raised its terrorism threat level to "severe," its second-highest level, days before London hosts major international meetings on how to deal with militancy in Afghanistan and Yemen. Britain's threat level had been labeled "severe" for several years before being lowered last summer to "substantial."
American officials say a U.S. air strike on Christmas Eve against suspected al Qaeda training camps is believed to have killed many, but not all, of a group of suicide bombers being trained in Yemen.
The man accused of attempting to explode a bomb on Northwest flight 253, Umar Farouk Abdulmutallab, told FBI agents there were a number of other people who trained with him in Yemen.
"There are others who are still out there who have been trained and who are clean skins -- that means people who we do not have a record of, people who may not look like al Qaeda terrorists, who may not be Arabs, and may not be men," said Clarke.
The alert comes during a week in which American law enforcement officials described an "unusually high" number of people on the no-fly list attempting to board flights to or in the United States.
