The Secretary of State's four-day trip comes amid new worries of the threat posed by North Korea.
"The rhetoric we are hearing from North Korea is simply unacceptable by any standard," Kerry said, according to Reuters. "We are all united in the fact that North Korea will not be accepted as a nuclear power."
A report by the Pentagon's Defense Intelligence Agency, which was disclosed in a congressional hearing on Thursday, claimed with "moderate confidence" that North Korea is capable to mounting a nuclear weapon on a missile.
The Pentagon has insisted since the disclosure that there is little evidence that the North Koreans have done so already, or that such a missile could be accurately fired.
Kerry echoed that sentiment in his remarks in Seoul.
“It is inaccurate to suggest that the DPRK [Democratic People's Republic of Korea] has fully tested, developed, and demonstrated capabilities that are articulated in that report,” Kerry said.
For more on Kerry's remarks, click here.