Panetta was expected to hold talks with Afghan President Hamid Karzai and Defense Secretary Abdul Rahim Wardak.
He was also to meet with commanders of the U.S.-led international force.
Panetta's trip is the second to the country since taking up his post in July.
The United States has announced the withdrawal of 10,000 of its 100,000 troops by the end of the year, while 23,000 others will leave the country by the end of September 2012.
The number corresponds to the 33,000 "surge" troops ordered in by President Barack Obama in late 2009 in a bid to reverse the Taliban insurgency, defeat Al-Qaeda and quicken an end to the war.
compiled from agency reports