U.S. President Barack Obama says the U.S. and the world will not "be cowed" by North Korea's nuclear threats.

Obama made the statement on Saturday during a lecture in Tokyo on U.S.’s Asia policies.

He urged North Korea to return to six-way nuclear talks with the United States, Japan, South Korea, China and Russia. He also stressed the need for international cooperation to repress the North’s nuclear weapons development.

Obama said that the U.S. supports the Doha Development Agenda, adding that the U.S. will work to ratify its free trade agreement with South Korea while also seeking to launch discussions on an Asia-Pacific trade accord.