Jim Mattis said that analysis continued on North Korea's most recent missile test, but he did not believe its (ICBM) posed an imminent threat to the U.S.
A North Korean ballistic missile was seen by the crew of several airliners last week raises concerns about the threat to civilian flights.
South Korea has detected signs that the North is preparing another missile launch.
North Korea warned Sunday that the United States will be "pouring gasoline on fire" by conducting.
The escalating threat arising from nuclear-armed North Korea's recent series of missile tests is prompting South Korea to beef up its military muscle.
American forces successfully tried out Sunday a missile interception system the US hopes to set up on the Korean peninsula.
North Korea said on Saturday it had conducted another successful test of an intercontinental ballistic missile (ICBM) that proved its ability to strike America's mainland.
New images of North Korea's main nuclear facility show that the isolated regime has apparently produced more plutonium for its weapons programme.
The US military said that a successful test of defenses against an attack from an intercontinental ballistic missile shows the US is staying ahead of the evolving ICBM threat.
The US military will try to intercept an intercontinental ballistic missile in a landmark test of a defense system.