"Clearly it is too early to say... if that is going to have any traction once the Olympics are over," Mattis said.
"In the midst of all of this, he (North Korean leader Kim Jong Un) ran a military parade that highlighted his ballistic missiles," Mattis told reporters on his plane en route to Rome on the first leg of a European tour.
"That's a very strange time if in fact he is trying to show a warming to a country that he has attacked repeatedly as an American puppet," said the US defence chief, who is to head next to Brussels and Munich.
After North Korea stalled for months before even announcing that it would take part in the Pyeongchang Olympics, the Games have turned into an occasion for a stunning rapprochement on the Korean peninsula.
On Sunday, South Korean President Moon Jae-in sat next to Kim's powerful sister at a concert in Seoul by musicians from Pyongyang.
Mattis also said he doubted that the sporting rapprochement would undermine US-South Korean ties.
"I know that people are watching for a wedge between South Korea and the United States. There is no wedge there," he said.
On Tuesday, the Pentagon chief will meet in Rome with 13 counterparts in the US-led coalition fighting the Islamic State group in Iraq and Syria.
"The fight is not over," Mattis said Sunday.
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