According to sources, McMaster met with officials from South Korea and Japan.
Secretary of State Rex Tillerson huddles Tuesday with nations that fought on America's side in the Korean War.
U.S. military officials explained that the military moves represent routine training and scheduled upgrades.
The rival Koreas took steps toward reducing their bitter animosity during rare talks Tuesday.
The talks come after Kim Jong-Un indicated that Pyongyang was willing to send a delegation to the Winter Games.
Mattis said there was little chance North Korea could use Olympics talks as a wedge to set Washington and Seoul at odds.
The 8th Army welcomed a new commander to lead its soldiers on the tense Korean Peninsula during a ceremony Friday.
The rival Koreas will sit down for their first formal talks in more than two years next week.
President Donald Trump approved the postponement in consultation with South Korean President Moon Jae-in.
Defense Secretary Mattis noted that military exercises have been rescheduled in the past for political or diplomatic reasons.