The official, who spoke on condition of anonymity due to the sensitive nature of the investigation, said the al Qaeda member is believed to be moving between Afghanistan's Nuristan and Kunar provinces, which border Pakistan.
The information was obtained by interrogators questioning a British suspect in the plot to blow up trans-Atlantic passenger planes, Rashid Rauf, who was arrested in eastern Pakistan and is regarded as a key figure in the foiled plot.
The information has already been shared with the British and coalition forces operating in Afghanistan, the official told The Associated Press.
The official did not provide the nationality of the wanted al Qaeda operative, but said he was a close aide to Egyptian-born al Qaeda No. 2 Ayman al-Zawahri. Detained terror suspects have told interrogators al-Zawahri likely approved the plan to blow up passenger planes leaving London for the United States, the official said.
Several hundred U.S. forces from the Fort Drum, N.Y.-based 10th Mountain Division are based in Kunar and Nuristan provinces hunting al Qaeda fighters and supporters of Afghan warlord Gulbuddin Hekmatyar.
Rauf was in touch with al-Zawahri through a courier from Afghanistan who would cross the porous, mountainous frontier separating Afghanistan and Pakistan to deliver messages, the official said.
The official said the Arab al Qaeda mastermind has developed links with several Pakistan-based militants, including Rauf.
U.S. officials were not immediately available for comment.
Rauf, who officials claim once belonged to the outlawed Pakistani militant group Jaish-e-Mohammed, is among at least seven people detained in Pakistan in a roundup that helped foil the plot.
Some 23 people have also been detained in Britain in connection with the plan, including Rauf's 22-year-old brother Tayib.
The Afghan-Pakistan border has long provided sanctuary for Afghan insurgents allied to Hekmatyar and the toppled Taliban regime, along with foreign fighters belonging to al Qaeda. Al-Zawahri and al Qaeda leader Osama bin Laden are believed to be hiding in the region.