WASHINGTON (Yonhap) -- U.S. Secretary of Defense Leon Panetta expressed optimism Thursday over talks with South Korea on its push to develop greater ballistic missile capability.

"I think we are making good progress," he said during a joint press conference with South Korean Defense Minister Kim Kwan-jin.

Secretary of State Hillary Clinton and South Korean Foreign Minister Kim Sung-hwan also joined in, describing the outcome of their one-day talks here.

The meeting marked the second "two-plus-two" session of the top diplomats and defense officials from the allies. The first one was held two years ago in Seoul.

Panetta said he hopes for a conclusion to one of the most sensitive issues between the two sides.

Seoul can't possess ballistic missiles with the range of more than 300 km (186 miles) and a payload heavier than 500 kg (1,102 pounds) under a 2001 agreement with Washington.

Korea's conservatives have been calling for an extension of the range to cover all of North Korea and reduce the missile gap between the two, still technically at war.

But Korea's defense minister, Kim, said the missile range issue was "not raised as agenda."

"Since working-level consultations are under way on this matter, it was not raised in today's talks," he said.

On a missile defense system, Kim said Seoul and Washington are conducting a joint study of what is most effective.

"Given the terrain of the Korean Peninsula, the most effective approach is a low-tier defense," he said.

A senior aide to Kim later told reporters that the minister was referring to views that South Korea needs a limited missile defense system just to cover direct threats to its territory.

Sen. Carl Levin, the chairman of the Senate Armed Services Committee, said earlier this week that he would not take issue with Korea's development of longer-range missiles if they are deployed in a "defense and non-threatening" way and at Seoul's own expense.