Chuck Hagel in South Korea for military talks
US Defence Secretary Chuck Hagel is in South Korea for talks, where the eventual transfer of military control to Seoul is expected to be a key issue.
Under the current alliance, the US has operational control over South Korean troops in the event of war.
Seoul was due to resume control in 2015, but appears to want an extension, given ongoing tensions with the North.
The two Koreas remain technically at war as the 1950-53 conflict ended in an armistice and not a peace treaty.
Mr Hagel, who is visiting South Korea for the first time since becoming defence secretary, said Seoul's military had become "much more sophisticated, much more capable".
"We're constantly re-evaluating each of our roles," he told reporters on Sunday during his flight to Seoul.
However, it was not the time "to make any final decision" on when to hand back military operational control to the South, he added.
The US assumed military operational control over South Korea during the Korean war.
South Korea resumed peacetime control in 1994. However, the US is still obliged to lead South Korea's military, as well as US troops deployed in South Korea, in the event of war.
South Korea was due to take over that role last year, but the date was pushed back to 2015 after North Korea's repeated nuclear and rocket tests, the BBC's Lucy Williamson in Seoul reports.
Tensions between the two Koreas rose earlier this year, after North Korea's third nuclear test in February.
Earlier this month, satellite imagery also suggested that North Korea had restarted a reactor at its Yongbyon nuclear facility, and tested a long-range rocket engine, a US think tank said.
Mr Hagel will meet South Korean president Park Geun-hye during his visit.
This is Mr Hagel's third trip to Asia this year. On Wednesday, he will head to Japan for talks.