US Vice President Mike Pence has put North Korea on notice, warning that recent US military strikes in Syria and Afghanistan showed the resolve of President Donald Trump should not be tested.

Pence and South Korean acting president Hwang Kyo-ahn, speaking after a failed missile test by North Korea and a huge display of missiles in Pyongyang, said they would move ahead with the early deployment to South Korea of the US THAAD missile-defence system.

Mr Pence was on the first stop of a four-nation Asia tour intended to show that the Trump administration is not turning its back on the increasingly volatile region.

"Just in the past two weeks, the world witnessed the strength and resolve of our new president in actions taken in Syria and Afghanistan," Pence said in an appearance with Mr Hwang yesterday.

"North Korea would do well not to test his resolve or the strength of the armed forces of the United States in this region," Mr Pence said.

The US Navy this month struck a Syrian airfield with 59 Tomahawk missiles after a chemical weapons attack. On Thursday, the US military dropped "the mother of all bombs", the largest non-nuclear device it has ever unleashed in combat, on caves and tunnels used by Islamic State in Afghanistan.

Tension has escalated sharply on the Korean peninsula amid repeated North Korean missile tests and concerns that Pyongyang may soon conduct a sixth nuclear bomb test.

Pyongyang has carried out missile and nuclear tests in defiance of UN sanctions, and has been working to develop a nuclear-topped missile that could strike the US mainland, although US officials say this capability is still several years away.

Senior North Korean officials on Monday reiterated recent rhetorical warnings that the situation on the Korean peninsula is "nearing the brink of war."

Kim Song Gyong, director general of the European Department of the Foreign Ministry in Pyongyang, told Reuters that if Washington made "the slightest movement" to make a nuclear strike on North Korea, Pyongyang would strike first and "destroy the aggressors without any mercy."

Kim went on to clarify that the approach to the Korean peninsula of a US aircraft carrier strike force, led by the nuclear powered USS Carl Vinson, would not be considered enough to constitute "the slightest movement".

North Korea's deputy representative to the United Nations, Kim In Ryong, accused Washington of creating "a situation where nuclear war could break out an any time" and said Pyongyang's next nuclear test would take place "at a time and at a place where our headquarters deems necessary."

At a White House Easter celebration, Trump was asked by a reporter if he had a message for North Korean leader Kim Jong Un, and replied: "Gotta behave."

Pence, whose father served in the 1950-53 Korean War, visited the border between North and South Korea earlier on Monday and said Washington would stand by its "iron-clad alliance" with South Korea.