"It is another sign of steady progress that the government of Afghanistan has announced the fourth tranche of the transition process," Xinhua quoted Panetta as saying in a statement Monday.
Earlier in the day, Karzai announced that Afghanistan had entered the second to last stage of the transition of security responsibility from NATO's International Security Assistance Force (ISAF) to Afghan security forces.
Afghanistan's security transition is scheduled to take place in five tranches, or stages. As part of the fourth tranche, 12 Afghan provinces entered the security transition process, ISAF officials said.
The country now has 23 of its 34 provinces under transition process.
US President Barack Obama has announced a withdrawal strategy that would see security responsibilities transferred to Afghans by the end of 2014.