WASHINGTON (REUTERS, AFP) - The US Missile Defence Agency and the Navy conducted a missile defence test off the coast of Hawaii and successfully intercepted a medium range ballistic missile, MDA said in a statement on Wednesday (Aug 30), as North Korea's firing of a ballistic missile over Japan could increase pressure on Washington to consider shooting down future test launches by the hermit regime.

The missile defence test, scheduled well in advance, was done from the USS John Paul Jones, a guide-missile destroyer, and comes a day after North Korea fired a ballistic missile over Japan.

The MDA said the test, which used Standard Missile-6 guided missiles, gives the naval component of the missile defence system higher ability to intercept ballistic missiles in their terminal phase.

"We are working closely with the fleet to develop this important new capability, and this was a key milestone," in advancing the capability to intercept missiles, said Lt. Gen. Sam Greaves, the director of the Missile Defence Agency, which is part of the US Defence Department.

"We will continue developing ballistic missile defense technologies to stay ahead of the threat as it evolves."

The test marks the second time that an SM-6 missile has successfully intercepted a medium-range ballistic missile target. The United States and Japan have been working together since 2006 to develop a variant of the Standard Missile-3, a ship-launched missile that operates as part of the Aegis system. Japan is seeking a land-based version as well.

North Korea said its missile launch on Tuesday was to counter US and South Korean military drills, and was a first step in military action in the Pacific to "contain" the US territory of Guam. The launch was condemned by the United Nations as an "outrageous" act.

Japan has been worried that the United States has so far declined to arm it with a powerful new radar, Reuters reported on Wednesday. Japan is seeking a land-based version of the Aegis ballistic missile defence system, operational by 2023, as a new layer of defence to help counter North Korea's missile advances.

More attention is likely to focus on the prospects for intercepting a missile in flight after North Korea on Tuesday conducted one of its boldest missile tests in years, one government official said.

Such a decision would not be taken lightly given tensions over North Korea's nuclear and missile programmes.

And while President Donald Trump has repeatedly vowed that"all options are on the table", there has been no sign of any quick policy shift in Washington toward direct US military action.

But Pyongyang's launch of an intermediate-range Hwasong-12 missile over Japan's northern Hokkaido island underscored how Trump's tough rhetoric, pursuit of sanctions and occasional shows of military force around the Korean peninsula have done little to deter North Korea's leader.

"Kim Jong Un has chosen to thumb his nose at the Americans and Japanese by conducting this test," said David Shear, former US assistant secretary of defence for East Asia.

US Defence Secretary Jim Mattis has already pledged that the military would shoot down any missile it deemed a danger to US or allied territory.

What is unclear is whether Washington would be prepared to use its multi-layered missile defence systems to intercept a missile like the one that overflew Japan but never directly threatened its territory.

Doing so would essentially be a US show of force rather than an act of self-defence. "I would think that in government deliberations that would likely be one of the options out on the table," Shear said.

Some analysts say there is a danger that North Korea would see it as an act of war and retaliate militarily with potentially devastating consequences for South Korea and Japan.

China, North Korea's neighbour and main trading partner, would also likely oppose such a direct US military response.

Experts say there is no guarantee that US missile defence systems, including Aegis ballistic missile defence ships in the region and Terminal High Altitude Area Defence (Thaad) systems based in Guam and South Korea, would hit their target, despite recent successful tests.

A failed attempt would be an embarrassment to the United States and could embolden North Korea, which this year has already conducted two tests of an intercontinental ballistic missile believed capable of hitting the U.S. mainland.

The United States has spent US$40 billion (S$54 billion) over 18 years on research and development into missile defence systems but they have never been put into operation under wartime conditions.

Mattis this month expressed confidence the US military could intercept a missile fired by North Korea if it was headed to Guam, after North Korea said it was developing a plan to launch four intermediate range missiles to land near the US territory.

If North Korea fired at the United States, the situation could quickly escalate to war, Mattis said.

Not everyone is convinced the US military can defend against North Korea's growing missile capability.

Some experts caution that US missile defences are now geared to shooting down one, or perhaps a small number, of incoming missiles.

If North Korea's technology and production keep advancing, US defences could be overwhelmed.

"If a shootdown fails, it would be embarrassing, though not terribly surprising," said Michael Elleman, a missile expert at the 38 North think tank in Washington. "Missile defence does not provide a shield that protects against missiles. Rather, it is like air defence; it is designed to minimise the damage an adversary can inflict," he said.

One US official said the military would be especially cautious about shooting down a North Korean missile that did not pose a direct threat because of the risk of civilian casualties if it were intercepted over Japan or South Korea, as well as difficulty in determining how Pyongyang might retaliate. The official spoke on condition of anonymity.

US military and intelligence officials warn North Korea could unleash a devastating barrage of missiles and artillery on Seoul and US bases in South Korea in response to any military attack.

Targeting of a North Korean missile in flight that did not endanger the United States or its allies could also raise legal questions. UN Security Council resolutions that ban Pyongyang's ballistic missile programs do not explicitly authorise such actions.

Japan also faces questions over the legality of shooting down missiles in its airspace but not aimed at Japan. Under legislation passed in 2015, Tokyo can exercise a limited right of collective self-defence, or militarily aiding an ally under attack, if it judges the threat to Japan as "existential".