US Defense Secretary Chuck Hagel late Saturday ruled out any quick decision on extending US wartime command over South Korea troops, a move authorities in Seoul are seeking.

"I don't think we'll be in a position to make any final decision," Hagel told reporters aboard the plane that was taking him to South Korea.

In case of war with North Korea, the alliance between Washington and Seoul currently calls for the US military commander to lead the 28,500 US troops deployed to the country, as well as South Korea's 640,000-strong force.

During peacetime however each side maintains operational command of their own troops.

South Korea agreed to take over wartime operational command of all troops starting in 2015, a decision that was already delayed from the 2012 target date.

Tensions with North Korea however have made Seoul cautious about that date, and the South has asked Washington to review the timing of the planned transition.

The question of when operational control should be passed to the South Koreans will feature prominently in Hagel's talks with his counterpart and President Park Geun-Hye.

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

The South Korean military has "become much more sophisticated, much more capable, much more qualified over the last 10 years in particular," Hagel said.

Any changes would "not at all subtract from or in any way weaken the United States commitment to the treaty obligations we have and we'll continue to have with the South Koreans," Hagel emphasized.

During the visit Hagel will attend celebrations commemorating the 60th anniversary of the alliance between the two countries, a bond that dates back to the armistice that ended the Korean War in 1953.

On Monday Hagel is scheduled to tour the demilitarized zone between the South and North with his counterpart, Kim Kwan-Jin and watch joint US-South Korean joint exercises.

On Tuesday, he is due to attend a parade marking the country's armed forces day and to hold talks with Kim.