A U.S. State Department official says the plan to return wartime operational control (OPCON) to South Korea by the end of 2015 remains on track but it's not certain if the two sides will be entirely ready by the target date.

Joseph Yun, acting assistant secretary of state for East Asian and Pacific affairs, told a congressional hearing Thursday that the current plan is to transfer OPCON by 2015 but a lot of work needs to be done.

Yun's comments are viewed as him stressing that Seoul and Washington need to establish comprehensive measures to strengthen their combined defense capability as they prepare for the scheduled transfer.

To make sure preparations stay on track, the two sides will check their basic and complete operational capabilities during the joint Key Resolve exercise in March and the Ulchi Freedom Guardian exercise in August next year.