U.S. Secretary of Defense Chuck Hagel said it's premature to conclude a new deal with South Korea on its request for another delay in the transition of operational control (OPCON) of its troops in time of war.

"I don't think we'll be in a position to make any final decision," Hagel was quoted as telling reporters aboard a military plane heading to South Korea.

He added, "I don't think anything is fixed for decades at a time. There's always a review and realignment of responsibilities and forces," he added.

Hagel is making a four-day trip to South Korea this week, an unprecedentedly long visit there for a Pentagon chief.

The OPCON transfer timeline, along with North Korea's threats, is expected to be high on the agenda when he holds an annual Security Consultative Meeting with the South's Defense Minister Kim Kwan-jin on Wednesday.

Hagel strongly indicated the U.S. is taking a prudent approach toward Seoul's request for pushing back the agreed date for the OPCON transition, which is December 2015.

U.S. military officials point out South Korea's military capability has considerably improved in recent years.

But South Korean defense policymakers say they need more time to prepare for taking over the "wartime OPCON," given the North's increased military threats, especially after its February nuclear test.

South Korea handed over its operational command to the U.S.-led U.N. troops shortly after the outbreak of the 1950-53 Korean War.

Seoul regained peacetime OPCON in 1994.

It initially agreed to take back OPCON in the event of war in 2012, but the previous South Korean administration suggested a delay following the North's deadly torpedo attack on a South Korean warship in 2010. Washington accepted the offer.

Hagel, meanwhile, underscored the importance of bolstering Washington's ties with Asia-Pacific countries amid its rebalancing toward the region and federal budget constraints.

"It's important that the United States pay attention, and continue to strengthen and enhance our relationship here," he said.

It's his third trip to Asia since taking office in February.

His visit to Seoul, the first as defense secretary, comes as the two nations mark the 60th anniversary of their alliance this year.

He plans to leave for Tokyo on Wednesday, where he will hold the so-called two-plus-two talks joined by Secretary of State John Kerry. (Yonhap News)