William McKinney, a retired Army colonel who spent more than 40 years involved in U.S. -Korea military relations and planning, said the U.S. should consider suspending larger exercises - provided it gets something in return.
The military exercises always take place at the end of August, therefore they could be seen as a routine affair.
Leader Kim Jong-Un last week delayed the Guam strike plan, but warned it could go ahead depending on Washington's next move.
China, North Korea's primary trade partner, has called for calm during the recent weeks, expressing frustration with both the North Korean regime over its repeated nuclear and missile tests and the Trump administration for their sabre rattling that it sees as unnecessarily escalating tensions in the region.
The exercise is defensive in nature, Mattis said, noting that joint exercises have been going on for decades.
The Ulchi-Freedom Guardian is a military drill which sees the US and South Korea join forces in a string of exercises.
Other members of the United Nations Command - Australia, Canada, Colombia, Denmark, New Zealand, the Netherlands and Britain - have also sent their forces but the drills are majorly led by the US and South Korea.More news: Powerball jackpot increases to $535 million
The annual drill has been held between the two countries since the 1970s and about 17,500 United States troops and 50,000 South Korean troops will participate in the exercises, which will last for about 10 days.
The Ulchi-Freedom Guardian military exercises routinely spark condemnation from North Korea.
"We call on all countries to redouble their efforts, including through implementation of agreed UN Security Council resolutions, to bring North Korea to its senses and end its reckless and unsafe threats to the peace of our region and the world", he added.
"The dual freeze approach is China's attempt at buck passing and moral equivalency between US actions and North Korean actions", Hass said.
On Sunday, the North's main state-run newspaper, Rodong Sinmun, likened the drills to an act of "throwing fuel onto fire" that would "worsen the situation".
Last year's Ulchi-Freedom Guardian event involved almost 80,000 troops in land, sea and air drills.
Kim would "watch a little more the foolish and stupid conduct of the Yankees", a North Korean statement said last week.
The 11-day exercise will be the first major exercise since North Korea test-fired two intercontinental ballistic missiles last month that appeared to bring much of the United States within range.