"Clearly, it is another manifestation of the problems posed by North Korea's efforts to acquire nuclear weapons," Bosworth told reporters in Seoul.
"If we are going to deal as we wish with the denuclearization of the Korean peninsula, this is an issue that will have to be clarified," he said.
Bosworth, who visited Beijing before coming to Seoul, was to leave for Tokyo later Sunday. Sung Kim, the chief U.S. nuclear negotiator who is traveling with Bosworth, is to return to Seoul on Tuesday to meet visiting Russian nuclear envoy Grigory Logvinov.
Bosworth said he and South Korean officials agreed "entirely" that the complete, verifiable denuclearization of the Korean peninsula remains their prime interest.
North Korea is believed to have processed enough plutonium for at least half a dozen bombs, and said Friday it is continuing to weaponize plutonium. Experts say uranium offers an easier way to make nuclear weapons.
The North pulled out of six-nation disarmament talks -- involving the two Koreas, the U.S., China, Russia and Japan -- in April to protest international criticism of a controversial rocket launch. It conducted its second nuclear test in May.
North Korea has said it will only talk one-on-one with the U.S.
"As we have indicated in the past, we are prepared to engage bilaterally as well with the North Koreans, but only in the context of the six-party process," Bosworth said.
Washington has shown no signs of easing pressure on North Korea through new U.N. sanctions aimed at punishing the North's latest nuclear test, despite a series of conciliatory gestures by the regime such as the release of two American journalists and five South Koreans detained in the country.