US Vice-President Mike Pence made a surprise visit to the demilitarised zone separating North and South Korea today, amid tensions in the region over Kim Jong Un's belligerent actions.

Pence landed at a US military base next to the demilitarised border with North Korea, a day after a failed missile launch by the North.

The demilitarised zone (DMZ) is a heavily mined, 4km-wide strip of land lined with barbed wire running across the Korean Peninsula, with soldiers on both sides in a continual eyeball-to-eyeball standoff.

Pence, whose father served in the 1950-53 Korean War, said he was humbled to be at the DMZ and hailed the alliance with South Korea. "It is a testament to the unshakeable bond between our people," he said.

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Pence is on the first leg of a four-country trip to Asia.

The US did not need to respond to North Korea's latest missile launch because it was a failure, the White House said.

As Pence was departing from Alaska - a day after a huge military parade in Pyongyang - North Korea launched a ballistic missile that exploded within a few seconds.

North Korea had been expected to do something provocative to mark the most important day on the regime's calendar, the April 15 anniversary of founder Kim Il Sung's birthday.

Tensions in the region had been running high amid expectations of a nuclear test or a missile launch, with President Trump vowing to "properly deal with" North Korea if China would not.

But a White House foreign policy adviser travelling with Pence said that the United States did not need to take action "to reinforce their failure".

North Korea has fired numerous medium-range missiles, as yesterday's appeared to be, so one more made little difference.

"If it had been a nuclear test, then other actions would have been taken from the U.S.," the adviser told reporters on the vice-president's plane.

Some analysts were puzzled by this.

"I'm not sure why failed missile tests, which are still banned by the UN Security Council, are considered less provocative," said Kent Boydston, a North Korea-focused research analyst at the Peterson Institute for International Economics. "The North Koreans know they may fail, but they improve their capabilities each time."

Indeed, while the latest missile exploded soon after launch, experts say North Korea is able to learn from its mistakes and hone its technology. It had repeated failures with other missiles, including the medium-range Musudan and a submarine-launched ballistic missile, before successfully firing both.

North Korea's behaviour will be the focus of Pence's trip to Asia, with a senior Administration official saying the Vice-President would be discussing "the belligerency of North Korea" at every stop.

After stopping in Seoul, Pence is to travel to Tokyo; Jakarta, Indonesia; and Sydney.

North Korea's missiles are a particularly complicated issue in South Korea.

The conservative government of former President Park Geun Hye, who was removed from office last month after her impeachment in December, had agreed to deploy the Terminal High Altitude Area Defence, or THAAD, antimissile system to defend against North Korea's rockets.

South Korea made this decision last July after much equivocation. China strongly objects to the THAAD battery - in particular its radar system, which Beijing worries will be used to peer into China.

Beijing has made its unhappiness known with a wide economic boycott that includes K-pop concerts, toilet-seat imports and tour group travel to South Korea.

Now, South Korea is heading towards a snap presidential election on May 9 to replace Park, and the THAAD system is a core issue.

Moon Jae In, the progressive candidate who had been leading in the polls, has promised to review the previous government's decision to host THAAD. Apparently sensing a worsening political environment, the US military sped up the deployment to try to get everything in place before the election.

But now Moon faces a strong challenge from a more centrist candidate, Ahn Cheol Soo, who has said that he will respect the Park Administration's agreement with the United States.

"It's a critical point for the defence of South Korea in recognising it's not an offensive weapon. It's there to prevent rockets slamming from the North Koreans," an American official said while previewing Pence's trip.

The complicating factor here is China. It wants the missile battery gone, and the United States wants China to crack down on North Korea. Although Trump had a cordial summit with his Chinese counterpart, Xi Jinping, in Florida this month, the North Korea issue threatens to drive a wedge between them.

China on Friday warned that "storm clouds" were gathering.

Foreign Minister Wang Yi urged the United States and North Korea not to push their recriminations to a point of no return and allow war to break out on the peninsula.

Yesterday, China's top diplomat, Yang Jiechi, spoke to Secretary of State Rex Tillerson by telephone, and the two "exchanged views on the current situation on the Korean Peninsula," China's Foreign Ministry said in a statement.

China said it cut off coal imports from North Korea in February, in accordance with UN Security Council sanctions.

The US Navy sent a strike group to Korean waters last week, but defence officials have said tougher sanctions and pressure was still the favoured approach to dealing with North Korea.

"We've got a range of options, both militarily, diplomatic and others . . . at disposal for the president should he choose to use them," the White House foreign policy adviser told reporters on Air Force Two. "But for this particular case [of the failed missile launch] . . . we don't need to expend any resources against that."

Tensions are expected to continue for several weeks at least.

Large-scale annual exercises between the South Korean and U.S. militaries will continue until the end of April, and North Korea will mark another important date - the anniversary of the foundation of the Korean People's Army - on April 25.

- additional reporting AAP