The nominee to become the new U.S. ambassador to South Korea said Thursday the allies still have doubts over North Korea's seriousness about dialogue despite its recent peace gestures.

 "We are not convinced that they really are ready to return to serious diplomacy and negotiations. This is why, I think, Seoul and Washington have both been very cautious in just rushing back to the negotiating table," Sung Kim said at a Senate confirmation hearing.

He pointed out a typical cycle of North Korean provocations and short-lived negotiations over the past few years.

"I think the North Koreans need to prove that they will in fact be a serious partner when the negotiations resume," he said in contrast to expectations of an imminent breakthrough.

The State Department's top Korea expert, Kim was nominated by President Barack Obama in June to serve as his top envoy in Seoul.

The 51-year-old has spent most of his diplomatic career dealing with Korean affairs. He has worked as special envoy to the six-way talks on North Korea's nuclear program since 2008. He traveled to the North several times for talks with officials there.

If confirmed, he would become the first Korean-born American ambassador to South Korea, replacing Kathleen Stephens.

In spite of his long experience with North Korea, Kim said it is hard to come up with a "rational explanation" for its deadly attacks on South Korea last year.

If Pyongyang launched the attacks to help facilitate a power succession process, he said, it was a mistake.

"If they want the succession process to move forward smoothly, they should focus on responsible behavior, living up to international obligations and commitments, and refraining from provocative actions. And beginning meaningful dialogue with the South," he stressed.

He said he agrees that China has a "unique responsibility" to make sure North Korea does so, both as the host of the six-party talks and given its relationship with the North.

Reading a prepared statement earlier, Kim vowed to serve as a bridge between the allies in resolving the North Korean nuclear crisis and countering its belligerence.

"Having focused on this much of the past few years, I hope to continue to contribute to our common efforts to achieve the verifiable denuclearization of the Korean Peninsula in a peaceful manner, and better lives for the long-suffering people of North Korea," he said. "If confirmed, I look forward to coordinating closely on negotiating strategy as well as efforts to deter provocative actions by North Korea."

On the pending South Korea-U.S. free trade agreement, he placed emphasis on expected benefits for Americans such as more exports and jobs.

"I would say that there is really no reason why we should not implement this agreement," he said.

He also lent a personal note to the nomination.

"When my parents brought me to the United States some 35 years ago, they could not have imagined that I would have the opportunity to serve as the first Korean-American ambassador to the Republic of Korea," he said, using South Korea's official name.

 Kim immigrated to the U.S. in the 1970s and obtained U.S. citizenship in 1980. He is fluent in Korean as well.

He headed the Office of Korean Affairs at the department for two years from 2006. He worked at the U.S. Embassy in Seoul as political-military unit chief from 2002-06.

Thursday's hearing for Kim at the Senate Committee on Foreign Relations lasted just 40 minutes, as Sen. Jim Webb (D-VA) was the only questioner. Other senators at the committee were absent because they were busy with debt-ceiling talks and other pending matters, according to a congressional source.

Webb, chairman of the Senate Subcommittee on East Asian and Pacific Affairs, presided over the hearing.

In the face of post-hearing questions from South Korean correspondents, the nominee said he was honored and looking forward to a swift confirmation process.

Speaking to Yonhap News Agency, the senator was optimistic.

"I hope we can get this done by the August break (of Congress), hopefully within this week," he said, adding he expects a smooth process in general.

In his opening remarks in the hearing, meanwhile, Webb emphasized the significance of Kim's upcoming role in Seoul from a geopolitical point of view.

Northeast Asia is the only place in the world where the interests of the U.S. China, Russia, and Japan directly intersect, he noted.

"And in the middle of this is a divided Korean Peninsula with an erratic, volatile regime on one end and a key strategic ally, economic partner and democratic nation on the other," Webb added.

"It's important to remember that South Korea is the focal point for maintaining stability in this region, even as we work together to bring a lasting peace to this entire area." (Yonhap News)