US President Donald Trump said he would not rule out the use of military force against North Korea, as Japan sent its biggest warship since WWII to protect a US supply ship.

When asked at during a factory tour before his rally in Harrisburg, Pennsylvania, to mark his 100th day in office what his message on North Korea is, Trump told reporters: “You’ll soon find out, won’t you?”

Asked whether that meant military action, Trump said: “You’ll soon find out.”

His comments came as Japan dispatched its biggest warship since World War II to protect a US supply ship, as tensions mount in the region over North Korea.

The helicopter carrier Izumo will leave the mother port of Yokosuka, south of Tokyo, on Monday and join the US supply ship to escort it further into the western Pacific, the leading Asahi Shimbun daily and Jiji Press reported citing unnamed government sources.

It will be the first deployment -- outside of troop exercises --to protect the US fleet after Prime Minister Shinzo Abe expanded the country’s military capabilities in 2015, though they remain restricted under Japan’s pacifist constitution.

The US supply ship is expected to support America’s naval fleet in the Pacific, possibly including the aircraft carrier USS Carl Vinson, which remains on high alert over North Korea’s ballistic missile firings, the reports said.

Japanese naval officials declined to comment on the reports. Earlier this week, the US carrier had joint drills with Japan’s naval forces. The Carl Vinson arrived in the Sea of Japan and kicked off a joint drill with the South Korean navy on Saturday, hours after North Korea launched a ballistic missile in apparent defiance of the US.

North Korea’s state media has said the North’s military is capable of sinking the US aircraft carrier with a single strike.

The latest missile launch, which South Korea said was a failure, ratchets up tensions on the Korean peninsula, with Washington and Pyongyang locked in an ever-tighter spiral of threat, counter-threat and escalating military preparedness.

US President Donald Trump, who has warned of a “major conflict” with North Korean leader Kim Jong-Un’s regime, said the latest test was a pointed snub to China -- the North’s main ally and economic lifeline.

“North Korea disrespected the wishes of China & its highly respected President when it launched, though unsuccessfully, a missile today. Bad!” Trump tweeted

US WILL PAY FOR MISSILE SYSTEM

South Korea says Washington has reaffirmed it will shoulder the cost of deploying the THAAD anti-missile system, days after US President Donald Trump said Seoul should pay for the $US1billion ($A1.3 billion) system designed to defend against nuclear-armed North Korea.

In a telephone call on Sunday, Trump’s national security adviser, HR McMaster, reassured his South Korean counterpart, Kim Kwan-jin, that the US alliance with South Korea was its top priority in the Asia-Pacific region, the South’s presidential office said.

The conversation followed another North Korean missile test-launch on Saturday which Washington and Seoul said was unsuccessful, but which drew widespread international condemnation.

Trump, asked about his message to North Korea after the latest missile test, told reporters: “You’ll soon find out”, but did not elaborate on what the US response would be.

Trump’s comments in an interview with Reuters on Thursday that he wanted Seoul to pay for the THAAD deployment perplexed South Koreans and raised questions about his commitment to the two countries’ alliance.

South Korean officials responded that the cost was for Washington to bear, under the bilateral agreement.

“National Security Adviser HR McMaster explained that the recent statements by President Trump were made in a general context, in line with the US public expectations on defence cost burden-sharing with allies,” South Korea’s Blue House said in a statement, adding that McMaster requested the call. Major elements of the advanced Terminal High Altitude Area Defense (THAAD) system were moved into the planned site in Seonjgu, in the south of the country, this week.

South Korea and the US say the sole purpose of THAAD is to guard against North Korean missiles. China says its powerful radar can penetrate its territory and undermine its security and spoke out against it again this week. The North has been conducting missile and nuclear weapons related activities at an unprecedented rate and is believed to have made progress in developing intermediate-range and submarine-launched missiles.

US JOINT DRILLS WITH S KOREA

THE US aircraft carrier USS Carl Vinson have kicked off a joint drill with the South Korean navy, officials said, as tensions rose over North Korea’s latest test-fire of a ballistic missile.

“Immediately after the aircraft carrier arrived in the Sea of Japan, South Korea and the US strike forces launched a drill from 9am GMT (7pm Saturday AEST) Saturday”, a defence ministry spokesman said.

The drill came hours after the North launched a ballistic missile in apparent defiance of a concerted US push for tougher international sanctions to curb Pyongyang’s nuclear weapons ambitions.

North Korea disrespected the wishes of China & its highly respected President when it launched, though unsuccessfully, a missile today. Bad! — Donald J. Trump (@realDonaldTrump) April 28, 2017

The drill aimed to verify the allies’ capability to track and intercept enemy ballistic missiles, the spokesman said.

He declined to clarify how long the drill would last, but Yonhap news agency said it was expected to continue until sometime next week.

The drill will also include a live-fire exercise and anti-submarine manoeuvres, the spokesman added.

It would mark the USS Carl Vinson’s second operation in South Korean waters in less than two months amid heightened military tensions in Korea. In March, the aircraft carrier carried out annual joint drills.

Earlier this week, the US carrier had joint drills with Japan’s naval forces. North Korea’s state media has said the North’s military is capable of sinking the aircraft carrier with a single strike.

The latest missile launch, which South Korea said was a failure, came after US Secretary of State Rex Tillerson warned the UN Security Council of “catastrophic consequences” if the international community — most notably China — failed to pressure the North into abandoning its weapons program.

Military options for dealing with the North were still “on the table”, Tillerson warned in his first address to the UN body.

The launch ratchets up tensions on the Korean peninsula, with Washington and Pyongyang locked in an ever-tighter spiral of threat, counter-threat and escalating military preparedness.

US President Donald Trump, who has warned of a “major conflict” with North Korean leader Kim Jong-un’s regime, said the latest test was a pointed snub to China — the North’s main ally and economic lifeline.

“North Korea disrespected the wishes of China & its highly respected President when it launched, though unsuccessfully, a missile today. Bad!” Trump tweeted.