Japan's biggest warship Izumo departed from Yokosuka base on Monday to help supply and protect the United States' largest aircraft carrier USS Carl Vinson amid heightened tension on the Korean Peninsula.
The move came as North Korea suggested the same day that it will continue its nuclear weapons tests, saying it will bolster its nuclear force in the face of what it calls US aggression and hysteria.
In an apparent show of solidarity with Washington, Japanese Defence Minister Tomomi Inada ordered the helicopter carrier Izumo on a mission to assist US naval ships.
The deployment is the first of its kind in the history of Japan's Self-Defence Forces. Critics say it violates Japan's pacifist constitution. However, the government of Prime Minister Shinzo Abe in 2016 changed the definition of self-defence to include the protection of Japan's allies and provide logistical support to them.
The USS Carl Vinson carrier strike group was believed to be in waters near the Korean Peninsula, amid signs North Korea could test-fire more missiles or conduct its sixth nuclear test.
The 249 metre-long (816.93 feet) Izumo is as large as Japan's World War Two-era carriers and can operate up to nine helicopters.
Trump has said a "major, major conflict" with North Korea is possible over its nuclear and ballistic missile programmes. China said last week the situation on the Korean peninsula could escalate or slip out of control.
Reclusive North Korea has carried out five nuclear tests and a series of missile tests in defiance of UN Security Council and unilateral resolutions. It has been conducting such tests at an unprecedented rate and is believed to have made progress in developing intermediate-range and submarine-launched missiles.
It test-launched a missile on Saturday which Washington and Seoul said was unsuccessful, but which nevertheless drew widespread international condemnation.
Trump on North Korea's "smart cookie"
North Korea tested a "small missile" Trump said over the weekend in an interview on the CBS programme Face the Nation. "Well, I didn't say, 'Don't test a missile.' He's going to have to do what he has to do. But he understands we're not going to be very happy," Trump said about North Korean leader Kim Jong-un.
"People are saying, "Is he sane?" I have no idea. I can tell you this, and a lot of people don't like when I say it, but he was a young man of 26 or 27 when he took over from his father, when his father died…," Trump added.
"And at a very young age, he was able to assume power. A lot of people, I'm sure, tried to take that power away, whether it was his uncle or anybody else … So obviously, he's a pretty smart cookie."
Trump also spent his weekend firming up support in Southeast Asia to rein in North Korea.
The US president was speaking to the leaders of Thailand and Singapore on Sunday in an effort to keep diplomatic and economic pressure on Pyongyang, just two days after what appears to be yet another failed missile test.
In separate phone calls about the North's nuclear program, Trump also asked both prime ministers to visit Washington.
Duterte in DC
That invitation was extended to Rodrigo Duterte on Saturday, despite the Philippines president being widely criticised by human rights groups for his war on drugs, that's killed more than 8,000 people.
The White House on Sunday defended Trump's decision to invite Duterte to Washington, in what it describes as a "very friendly" phone call, saying Manila's cooperation is needed to counter the growing threat from the North.