Observers have suggested the strikes against Syrian President Bashar al-Assad could strengthen Trump's hand in talks, as they showed Kim that the West has the political will to back up its rhetoric -- as well as the capability for precise strikes launched from afar.

"This is an action that was taken against weapons of mass destruction, and I think this gave a certain message to North Korea as well," Defense Minister Itsunori Onodera said through an interpreter.

Mattis said last week's strikes on three facilities tied to Assad's chemical weapons program had garnered broad international backing.

"Let me address Assad, should he ignore the international community," Mattis said.

"There has been full support for that regrettable but necessary attack on his research and engineering part of his weapons program -- weapons of mass destruction. He would be ill-advised to ignore the international community's statement, and we stand ready to address anything in the future."

Onodera was at the Pentagon for a meeting with Mattis, who said the US-Japan alliance was "the cornerstone for peace and security" throughout the Pacific region.

"Together we are carefully reviewing a possible new path to peace, and at the same time we remain vigilant," he said, referring to North Korea.

Onodera said the US and Japan must work "synergetically" along with the international community to "make North Korea abandon all weapons of mass destruction and ballistic missile program in a complete, verifiable and irreversible manner."

Trump and Kim are expected to have talks around Kim's nuclear program in late May or June. The location of what would be a historic summit has not yet been disclosed.