Tens of thousands of troops are taking part in the "Ulchi Freedom Guardian" joint military drills, a largely computer- simulated exercise that runs for two weeks in the South.
They are described as defensive, but nuclear-armed Pyongyang views them as a highly provocative rehearsal for invasion.
The drills come during a standoff on the peninsula since Pyongyang tested two intercontinental ballistic missiles (ICBM) last month that appeared to bring much of the US within range.
Pyongyang threatened to fire a salvo of missiles towards the US territory of Guam - a plan that leader Kim Jong-Un last week delayed, but warned could go ahead depending on Washington's next move.
While the allies are pushing ahead with the exercises that date back to 1976, around 17,500 US troops will participate in the drills - a cutback from last year.
South Korean media reports have said the US was considering scrapping a plan to bring in two aircraft carriers to the peninsula.
"This right now is an exercise to make certain that we're ready to defend South Korea and our allies over there," Mattis told reporters aboard an aircraft flying to Amman, Jordan.
US Pacific Command chief Admiral Harry Harris arrived in the South yesterday to inspect the exercises and discuss growing North Korean nuclear and missile threats.
On the eve of the UFG drills, North Korea said the US was "pouring gasoline on fire".
Washington was "mistaken" to think that such a war would take place on "somebody else's doorstep far away from them across the Pacific", it added.
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