South Korean President Moon talks North Korea's nuclear activities, BTS' new diplomacy role

South Korean President Moon Jae-in said Tuesday in an exclusive interview it was "concerning" if North Korea has resumed its nuclear activities "in earnest" and stressed the importance of reactivating talks between North Korea and the United States, as well as inter-Korea talks.

"I believe that we need to have North Korea understand that dialogue and diplomacy are the only way to achieve peace on the Korean Peninsula," Moon told ABC News anchor Juju Chang.

Moon is attending the United Nations General Assembly in New York City this week to talk about climate change.

The International Atomic Energy Agency reported last month that North Korea appears to have restarted the operation of its main nuclear reactor used to produce weapons fuels, as North Korea openly threatens to enlarge its nuclear arsenal amid dormant nuclear diplomacy with the United States.

When asked about North Korea's nuclear program, which is going "full steam ahead" according to U.N. atomic watchdog chief Rafael Grossi, Moon said while North Korea has been "intensifying tension, launching missiles and conducting other activities, it is of great relief that it has kept good on its moratorium on nuclear tests and ICBM launches."

Moon was also asked about North Korea's criticism of the United States' decision to provide nuclear-powered submarines to Australia. North Korean state media quoted an unidentified North Korean Foreign Ministry official who called the arrangement, made between the U.S., Britain and Australia, an "extremely" dangerous move, one that could set off a nuclear arms race.

Moon conceded it is a "great pity that the Korean Peninsula still is living in the era of the Cold War," adding that while "remarkable changes" have taken place during his time in office, they have "yet to consolidate peace on the Korean Peninsula."

Moon said he believes there is a "possibility of resuming talks" and thus finding "a way, a pathway to the solution."

Tensions have been high between North and South Korea. Both countries have recently tested ballistic missiles. This came as a stark contrast to their 2018 agreement when the two rival nations vowed to denuclearize the peninsula and end the long war between them.

ABC News also sat down with K-pop sensation BTS exclusively to talk about the band's new official role as South Korea's presidential envoys for public diplomacy.

As part of their new role, BTS attended the U.N. Global Assembly and performed at the United Nations. Their new video for "Permission to Dance," which was filmed inside and around the U.N., has already racked up millions of views online. BTS said it is bringing a message of hope and community, talking about the importance of COVID-19 vaccines and sustainability.

The exclusive interviews with Moon and BTS will air on Sept. 24, starting with "Good Morning America."