President Barack Obama's nominee to be the new U.S. ambassador to South Korea says he does not believe North Korea is ready to resume nuclear talks.

Sung Kim said at his nomination hearing before the Senate Thursday that the North's recent actions show it is not interested in negotiations and serious diplomacy.

Kim was U.S. special envoy to the six-party talks with the North before they collapsed in 2008. He said it is very hard to determine exactly what North Korea thinks. Kim said it was not a rational decision for the North to sink a South Korean ship and shell a South Korean island last year.

Kim also called on the White House and Congress to complete work on the U.S.-South Korean free trade agreement. He called both economies mature enough to make such a deal necessary.

A disagreement between President Obama and Republicans over aid to workers has held up final approval.

If the Senate approves Kim's nomination, he would be the first Korean-born U.S. ambassador to South Korea.

Kim and his parents came to the United States 35 years ago.

The United States and its partners in talks with North Korea — China, Japan, Russia, and South Korea — want North Korea to give up its nuclear weapons program.