Mattis also confirmed that he and his South Korean counterpart had recently discussed the possibility of putting USA nuclear weapons back into South Korea, an option that has been raised publicly by some South Korean politicians.
In a phone call the two men discussed "North Korea's continued defiance of the global community and its efforts to destabilize Northeast Asia", the White House said.
The U.S. has seen no need to shoot down North Korean missiles test-fired in Japan's direction, but a future missile launch that threatens U.S. or Japanese territory will "elicit a different response from us", Defense Secretary Jim Mattis said Monday.
The war games come amid heightened tensions in the Korean peninsula, where the United States and South Korea have increased their military activity in the wake of North Korea's testing of two intercontinental ballistic missiles as well as its sixth and most powerful nuclear test.
The Czech president reaffirmed his country's support for Seoul's efforts to denuclearize the North, and said his country will actively play its necessary role in the worldwide arena, including the European Union, Cheong Wa Dae said. It was the country's longest-ever test flight of a ballistic missile.
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The agency said it did not believe that the breach had involved personal information or that it would jeopardize the agency's activities.
Military options available to Trump range from non-lethal actions like a naval blockade aimed at enforcing sanctions to waging cyber attacks and positioning new US weaponry in South Korea, where the United States has 28,500 troops.
Pyongyang says it needs nuclear weapons to protect itself from United States forces.
China and Russian Federation have repeatedly called for a peaceful solution and talks to resolve the issue.
The US and South Korea are technically still at war with North Korea because the 1950-1953 Korean conflict ended with a truce and not a peace treaty.
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South Korean Defense Minister Song Young-moo told lawmakers on Monday that it is "not proper" to reintroduce USA nuclear weapons. North Korea has lived under threat from United States for nearly seven decades.
"The so-called "China's responsibility theory" is essentially moral kidnapping", the People's Daily said in a commentary.
Mattis discussed several aspects of the North Korea crisis in an impromptu exchange with reporters at the Pentagon, including the effect of worldwide economic sanctions and diplomatic pressure on North Korea.
In this photo provided by South Korea Defense Ministry, U.S. Air Force F-35B stealth fighter jets drop bombs as they fly over the Korean Peninsula during a joint drills, South Korea on Monday, Sept. 18, 2017.
China has faced criticism for failing to act decisively against Pyongyang after a string of nuclear and missile tests this summer; its latest coming last Friday over Japan.
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