The first batch of U.S. soldiers set to leave Afghanistan left earlier this week, beginning a drawdown of 10,000 U.S. troops scheduled to depart by year's end.
Some 650 U.S. Army soldiers left the northeastern province of Parwan on Wednesday, according to Lt. Commander Colette Murphy, a spokeswoman for the NATO-led International Security Assistance Force.
Military officials say this is the first group of soldiers whose redeployment will not be replaced by a new rotation of fresh troops.
The remainder of a 33,000 U.S. troop surge - which began in 2009 - are expected to return home by the end of 2012, U.S. President Barack Obama said in June.
The full drawdown is expected to take place by the end of 2014.
The announcement comes one day after Gen. David Petraeus - the outgoing top commander of the NATO coalition forces in Afghanistan - crossed into Pakistan with his successor Marine Lt. Gen. John Allen to meet with top Pakistani military brass.
The meeting - Petraeus' seventh trip to the neighboring country as ISAF commander - was part of an effort to "improve regional security," according to a NATO statement.
The United States is currently holding back $800 million in aid to Pakistan, President Barack Obama's chief of staff said Sunday.
Relations between the two nuclear powers soured recently after a U.S. raid on former al Qaeda leader Osama bin Laden's compound in May.
Pakistan later decided to reduce the number of visas for U.S. military trainers.
Tensions have also been strained by U.S. drone attacks in Pakistan's tribal areas that have sometimes resulted in civilian casualties.
Meanwhile, Petraeus' exit and the troop drawdown comes amid a rash of recent violence across Afghanistan, particularly in country's restive southern provinces.
On Thursday, a suicide bomber slipped into a Kandahar mosque and killed at least six people, wounding 15 others, a hospital official said.
The attack occurred as several high-ranking Afghan officials had gathered to remember President Hamid Karzai's half-brother, Ahmed Wali Karzai, the Kandahar provincial council chief and key powerbroker in the region, who was slain Tuesday by a long-time bodyguard during a gathering at his house.