U.S. Defense secretary Jim Mattis says he cannot imagine the united states ever accepting a nuclear North Korea.

He warned Pyongyang that its rapidly advancing nuclear and missile programs would undermine, not strengthen, its security.

Addressing a joint news conference along with his South Korean counterpart Ong Young-Moo in Seoul, Mattis said North Korea’s military is no match for the U.S.-South Korean alliance, and that diplomacy is most effective “When backed by credible military force.”

Earlier, both the defense chiefs chaired an annual security consultative meeting.

Mattis emphasized diplomatic efforts to resolve the North Korean missile and nuclear crisis as he stood at the tense and heavily fortified border between north and south.

Mattis arrived in South Korea for a two-day trip to the country as tensions with North Korea climb ahead of a visit to the region next month by President Donald Trump.