Asked if he thought North Korean leader Kim Jong Un was a rational actor, Mattis said he would leave it to those seeking the answer to figure out themselves.
If carried out, it would be the North's most provocative missile launch to date.
His comments come after the North Korean military threatened a missile strike near the US territory of Guam, in response to comments made by President Donald Trump and a Pentagon bomber operation in the region.
On Thursday, Mr. Mattis declined to comment whether he was taken aback by Mr. Trump's harsh rhetoric.
Earlier Wednesday, U.S. Secretary of State Rex Tillerson defended Trump's remarks.
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That impetus has only grown among administration officials amid reports this week that Pyongyang had built a nuclear warhead small enough to fit atop one of the country's long-range missiles.
'His view and the view of the administration is that the way to resolve the situation with North Korea ... is through these economic sanctions.
"The best way to show how (the threat) is widely gauged, is to look at last weekend when the UN Security Council voted unanimously to characterize this as a North Korean threat to the world's community", he said.
Meanwhile, France is urging all sides "to act responsibly and to de-escalate" tensions over North Korea's nuclear ambitions.
An attack would require more military assets than the US now has stationed in the Korean peninsula and east Asia.
The Northern Viper drills are one of the scheduled exercises that Japan's Self Defence Forces conducts regularly with their USA counterparts and are not a response to the latest tensions. Guam Gov. Eddie Baza Calvo says he coordinates with Homeland Security and the US military on preparedness procedures.
North Koreans holding national flags march during a parade to mark the 60th anniversary of the signing of a truce in the 1950-1953 Korean War at Kim Il-sung Square, in Pyongyang July 27, 2013.