Mattis issued a statement as he traveled to the West Coast.
Though dwarfed by the U.S.' military, North Korea's is still one of the largest in the world, with 1.2 million active service members, nuclear weapons, and tens of thousand of artillery at its disposal.
Despite the different tone Tillerson and others have taken on North Korea, Mr. Trump said Thursday there is no confusion in views in his administration.
The US Defense Department is hoping for a diplomatic solution that would halt North Korea's quest for a nuclear intercontinental ballistic missile (ICBM) capable of reaching the United States, but has military solutions ready if ordered to act, Secretary of Defense Jim Mattis said in a press briefing on Thursday.
"What the president is doing is sending a strong message to North Korea in language that Kim Jong Un would understand, because he doesn't seem to understand diplomatic language", Tillerson said.
Jose Mourinho ready to offer Luke Shaw for Danny Rose
The former Southampton defender made just 19 appearances in all competitions last campaign, and only eight the season before. Manchester United are keen on Rose, a move which would pose questions over the long term future of Luke Shaw .
On Tuesday, Trump warned North Korea not to make any more threats against the United States. Secretary of State Rex Tillerson also took a calmer stance on North Korea earlier this week after Mr. Trump's initial "fire and fury" comments, saying Americans shouldn't be unsettled by recent rhetoric about the crisis.
Speaking to reporters in Mountain View, California, Mattis said that USA efforts on North Korea were now focused on diplomacy, while stressing that his "mission" was to make sure Washington "is to have military options should they be needed".
The Northern Viper drills are one of the scheduled exercises that Japan's Self Defence Forces conducts regularly with their US counterparts and are not a response to the latest tensions.
President Donald Trump "was informed of the growing threat last December and on taking office his first orders to me emphasized the readiness of our ballistic missile defense and nuclear deterrent forces", Mattis said.
The two discussed responses to North Korean provocations and the security situation on the Korean peninsula, he said. Moon is likely to highlight his policy on North Korea on the holiday, the only one the two Koreas share. The island has become a talking point in the rhetoric coming from President Trump and North Korea.
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