In Tokyo, the Yomiuri Shimbun newspaper says Washington has set the transfer of nuclear materials to a third country as a "red line" in its nuclear standoff with North Korea.

Quoting a senior U.S. government official, the paper said Tuesday that Washington would immediately take action in the event Pyongyang crosses that line.

The reports said the official did not elaborate on what that action would be.

However, the paper said the concept of "red line" signifies a strong warning that Washington would not rule out military action.