U.S. Defense Secretary Leon Panetta says he is “skeptical” about the outcome of the latest round of direct U.S. talks with North Korea aimed at resuming stalled nuclear disarmament negotiations.

Panetta expressed his reservations to reporters Thursday after holding talks with South Korean officials in Seoul. He said Pyongyang remains “a serious threat” by engaging in provocations “that target innocent lives,” and in its continued pursuit of nuclear weapons in defiance of the international community.

Panetta's comments came hours after U.S. Assistant Secretary of State Kurt Campbell told reporters in Seoul progress was made in the recent talks, but added “there were no breakthroughs.”

Campbell made an unannounced stop in Seoul to brief his counterparts on the second round of direct talks between American and North Korean officials in Geneva.

The visits by Panetta and Campbell are part of a flurry of diplomatic activity in South Korea over the North Korea nuclear situation. Chinese Vice Premier Li Keqiang arrived in Seoul earlier this week, after a trip to Pyongyang and talks with North Korean leader Kim Jong-il.

Li told a group of South Korean business leaders that Beijing wants a resolution to the North Korean nuclear stalemate, and is also willing to play a constructive role to help the North and South settle their differences.

North Korea's foreign ministry said Thursday that meetings earlier this week with the U.S. in Geneva had been helpful and that all sides had agreed to further confidence-building talks. North Korea has previously said it wants to resume talks about its nuclear program, but without preconditions set by the United States and South Korea, including a suspension of its uranium-enrichment program.

The six-nation nuclear talks, involving the two Koreas, China, Japan, Russia and the United States, were aimed at convincing North Korea to abandon its nuclear program in exchange for massive economic aid. Pyongyang walked away from the talks in April 2009, and later conducted its second nuclear-weapons test and several long-range missile tests.