"I wouldn't read too much into it," Mattis said. "We don't know if it's a genuine olive branch."
Mattis also said US military exercises with South Korea are being delayed until after the Paralympics which end on March 18.
The proposed talks between North and South Korea, Mattis added, are the result of international pressure on Pyongyang.
Earlier in the day, US President Donald Trump praised his own role in the resumption of contacts between Pyongyang and Seoul and called the talks between the two states "a good thing."
US Ambassador to the UN Nikki Haley said on Tuesday that the United States would not endorse any talks that do not result in the ban of all nuclear weapons from the Korean Peninsula.