The news surfaced as U.S. Secretary of State John Kerry was in Kabul trying to negotiate the terms of a U.S.-Afghan security agreement that would govern the activities of American forces in Afghanistan after the NATO-led combat mission ends at the close of next year.
Harf said she did not have any details of how, when or where Mehsud was captured.
Arsallah Jamal, governor of Logar province in eastern Afghanistan, said Mehsud was captured a week ago as he was driving along a main highway in Mohammad Agha district. The road links the province with the Afghan capital, Kabul. Jamal said Mehsud was in a car with two or three other men when the U.S. military arrested him.
The Pakistani Taliban confirmed the capture but claimed Mehsud was seized Oct. 5 by the Afghan army at the Ghulam Khan border crossing in the eastern province of Khost.
He was returning from a meeting to discuss swapping Afghan prisoners for money, said Pakistani Taliban commanders and intelligence officials, speaking on condition of anonymity because they were not authorized to speak with the media.
The Pakistani intelligence officials said American forces seized Mehsud while he was with the Afghan army, and that they no longer know where he is.
Mehsud, believed to be around 30 years old, once served as Hakimullah Mehsud's driver but eventually became a trusted deputy. The two are not related. Mehsud is a common name in the region.
The U.S. military in Kabul referred all questions to the Defense Department in Washington. A Pentagon spokeswoman, Navy Cmdr. Elissa Smith, said the Pentagon had no comment on the report.
A U.S. defense official said Mehsud was being lawfully held by U.S. military forces in Afghanistan. A senior U.S. official said the arrest did not please Afghan President Hamid Karzai, who apparently saw it as a violation of Afghan sovereignty. Both officials spoke on condition of anonymity because they were not authorized to speak about the issue with the media.
There was no immediate comment from Karzai's office on the report.
The detention may have contributed to a series of emotional outbursts this week by Karzai, who alleged that the U.S. and NATO have inflicted suffering on the Afghan people and repeatedly have violated its sovereignty.
There were reports that talks over the past two weeks on a bilateral security agreement were delayed because of the incident. American and Afghan officials have been meeting in recent days to negotiate the final details of the deal.
Karzai has ruled out signing the U.S.-Afghan deal until disagreements over sovereignty issues are resolved. The U.S. wants a deal in October and is reportedly ready to abandon talks and leave Afghanistan at the end of 2014 if one is not quickly reached.
The U.S.-led international coalition plans to withdraw its troops by the end of 2014, and American and its allies are considering keeping a small residual force in the country to train and assist Afghan security forces and go after the remnants of al-Qaida.
But almost a year of negotiations on the deal have failed to yield an agreement and it's possible the two sides will never produce one.
Without the United States on board, it is unlikely NATO or any of its allies will keep troops in Afghanistan. Germany has already indicated it will not commit the 800 soldiers it has promised.
Karzai has called a tribal meeting of Afghan elders known as Loya Jirga for next month to discuss the state of the U.S.-Afghan agreement and advise him on what to do.
The Taliban, meanwhile, condemned a unanimous U.N. Security Council vote to extend the mandate of the NATO-led force in Afghanistan for the last time before it hands over total responsibility for security to Afghan forces at the end of next year.
The resolution adopted by the council said the situation in Afghanistan "still constitutes a threat to international peace and security."