"It has not yet shown to be a capable threat against us right now ... we're still doing the forensics analysis," Mattis told reporters at the Pentagon.
Last month, North Korea said it had successfully tested a new type of ICBM that could reach all of the US mainland and South Korea and US-based experts said data from the November 29 test appeared to support that.
Mattis did not elaborate on what was lacking in the test to show it was not a capable threat.
He said himself immediately after the test that the missile had gone higher than any previous North Korean launch and that it was part of a research and development effort "to continue building ballistic missiles that can threaten everywhere in the world, basically."
US-based experts, some of whom have been sceptical about past North Korean claims, said last month that data and photos from the test appeared to confirm North Korea had a missile of sufficient power to deliver a nuclear warhead anywhere in America.
But experts and US officials have said questions remain about whether it has a re-entry vehicle capable of protecting a nuclear warhead as it speeds toward its target and about the accuracy of its guidance systems.
Some US based experts believe North Korea could be only two or three tests away from being able to declare the missile operational.
US President Donald Trump vowed "it won't happen" after North Korean said in January is was in the final stages of developing a nuclear weapon capable of reaching the United States.
Trump has said all options are on the table, including military ones, although his administration has made clear it prefers a diplomatic solution.
"It continues to be a diplomatically led effort," Mattis said. "When we're ready to have conversations ... dialogues, that will be up to the president and secretary of state."
At a UN Security Council session on the crisis on Friday, Secretary of State Rex Tillerson urged North Korea to carry out a "sustained cessation" of weapons testing to allow the two countries to hold talks.