Pompeo, Mattis, Australian Foreign Minister Julie Bishop and Australian Defense Minister Marise Payne were scheduled to hold a news conference on Tuesday at the Hoover Institution at Stanford University - the second day of the gathering.
The annual meeting between the two countries always includes discussions about security.
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The U.S. Department of State said this year's talks would focus on threats in the Indo-Pacific region and cooperation to defeat Islamic State militants. China and North Korea were also on the table.
The U.S. and China are locked in a trade dispute playing out in the imposition of tariffs by the two nations. The U.S., meanwhile, is pushing North Korea to dismantle its nuclear and missile programs entirely.
Pompeo was recently in North Korea to continue talks. President Donald Trump met with North Korean leader Kim Jong Un at a historic summit in Singapore on June 12.
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North Korea appears to have started dismantling key facilities at its main satellite launch site in a step toward fulfilling a commitment it made at the summit.
But analysts have said that move alone would not realistically reduce North Korea's military capability or represent a material step toward denuclearization.
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