US Special Representative for Afghanistan Zalmay Khalilzad departed on Friday to travel to Kabul, Islamabad, Oslo and Sofia, the department said in a statement.
The United States is drawing down its troops in Afghanistan under an agreement struck in February with the Taliban.
The agreement aimed to pave the way for formal peace talks between the insurgents and the Afghan government, and Khalilzad's task is to try and bring both sides to the table.
Khalilzad plans to press for a deal on prisoner exchanges and a reduction in violence, two issues that have hampered progress toward starting peace talks.
"Although significant progress has been made on prisoner exchanges, the issue requires additional effort to fully resolve," the State Department said in its statement.On Wednesday, Khalilzad condemned an attack by Afghan government forces that killed 45 people, including civilians, in airstrikes against Taliban fighters in a western province bordering Iran.