America’s highest ranking military officer says North Korea poses a “very real” threat to South Korea.

Admiral Mike Mullen, the chairman of the U.S. Joint Chiefs of Staff, told reporters in Seoul Thursday the isolated regime has shown no signs of easing in its pursuit of nuclear weapons.

Mullen made the remarks after attending the installation of General James Thurman as commander of the more than 28,000 U.S. forces in the country. Thurman, who succeeds General Walter Sharp, said the U.S.-South Korean alliance “stands ready to counter any provocation intended to destabilize the Korean peninsula.”

Mullen traveled to Seoul after a four-day to visit to China, in which he was given rare access to that country’s nuclear missile and submarine bases.

He visited a submarine base Wednesday in Shangdong province, east of Beijing, as well as an air base housing China’s advanced Su-27 jet fighters. He also visited the Second Artillery Force, which commands China’s nuclear arsenal.

The state-run China Daily newspaper said the visits were arranged “to highlight openness, transparency and self-confidence of China’s military power.” It said the visits reciprocated a trip in May by Chinese armed forces commander Chen Bingde to the largest U.S. naval base.

China’s official Xinhua news agency quoted Mullen as saying his visit had helped to ease some of his concerns about a lack of transparency regarding China’s military expansion. He said he regarded the exchanges as “a very positive step.”