US Secretary of State John Kerry reiterated on Friday in Seoul that the Democratic People's Republic of Korea (DPRK) will not be accepted as a nuclear power.

"We are all united in the fact that North Korea (DPRK) will not be accepted as a nuclear power," Kerry told a joint press conference with his South Korean counterpart Yun Byung-se. "The rhetoric that we're hearing from North Korea is unacceptable by any standard."

Kerry stressed that the US was fully prepared to "defend our allies and defend ourselves," repeating his comments made 10 days ago after talks with South Korean Foreign Minister Yun Byung-se in Washington.

Kerry arrived in Seoul on Friday afternoon on a four-day Asian tour that will also take him to China and Japan. The top American diplomat will fly to Beijing on Saturday.

Kerry made his first Asian trip as Secretary of State amid escalating tensions in the Korean Peninsula since the DPRK conducted its third nuclear test on Feb. 12.

Pyongyang has prepared to launch its new medium-range ballistic missile at any moment to fight against provocations of the U.S. and its allies.

Meanwhile, South Korean defense ministry spokesman said that the DPRK was estimated to be on its way to miniaturizing the nuclear warheads though he expressed doubts over the DPRK's capability to make small and light nuclear warheads that can be mounted on ballistic missiles.

"(The DPRK) has a significantly high level of technology on nuclear weapons and missiles. But making (nuclear devices) small, light and diversified was analyzed to take much time," Yun told reporters.