The Marshall Islands Monday painted a vivid picture of the horrors of nuclear war decades after some of its atolls were vaporised in atomic tests, in an unusual legal battle against the nuclear arms race.

The small Pacific island nation opened a case against India, Pakistan and Britain in a David-versus-Goliath fight before the UN's highest tribunal based in The Hague.

The Marshall Islands aims to shine a new spotlight on the global nuclear threat, its lawyers and representatives said Monday, as they remembered apocalyptic scenes after US-led tests in the 1950s.

"Several islands in my country were vaporised and others are estimated to remain uninhabitable for thousands of years," Tony deBrum, a Marshall Islands government minister, told the court.

He remembered in particular the "Castle Bravo" test of March 1, 1954, which he witnessed as a nine-year-old while fishing with his grandfather in an atoll, some 200 kilometres (125 miles) from the blast's epicentre.

"The entire sky turned blood red," deBrum recalled when US scientists exploded a hydrogen bomb, with a force 1,000 times stronger than the device dropped on Hiroshima in 1945.

"Many died, or suffered birth defects never before seen and cancers as a result of contamination."

Judges at the International Court of Justice are holding a series of hearings over the next week-and-a-half to decide whether it is indeed competent to hear the lawsuits brought against India and Pakistan.

A third hearing against Britain, scheduled to start on Wednesday, will be devoted to "preliminary objections" raised by London.

The Marshall Islands alleges that despite their suffering, the world's nuclear powers have failed to comply with the terms of the 1968 nuclear Non-Proliferation Treaty (NPT).

They were "not fulfilling their obligations with respect to the cessation of the nuclear arms race at an early date and to nuclear disarmament."

Britain has signed the NPT, but not India and Pakistan.

The hearings are starting just days after North Korean leader Kim Jong-Un Friday ordered the country's nuclear arsenal to be readied for pre-emptive use at any time, causing global concern.

- Pressure to cut arsenals -

The Marshall Islands had sought to bring a case against nine countries: Britain, China, France, India, Israel, North Korea, Pakistan, Russia, and the United States.

Israel has never admitted to having nuclear weapons.

And the ICJ has only admitted three cases against Britain, India and Pakistan because they already recognised the ICJ's authority.

Despite not having signed the NPT, India and Pakistan had an obligation under "customary international law" to negotiate and eventually reduce their nuclear arsenals, Majuro's lawyers insisted.

"Contrary to the obligation to pursue in good faith negotiations on nuclear disarmament... India's conduct includes the quantitative build-up and improvement of its nuclear arsenal," deBrum said.

India's representatives declined to comment Monday and are to argue their case on Thursday, with follow-up hearings next week.

In 1996, the ICJ in another case issued a non-binding advisory opinion in which it urged the world's nuclear powers to negotiate and reduce their nuclear stockpiles.

But two decades later, very little progress has been made to cut the nuclear threat, observers attending the case told AFP.

The Marshall Islands therefore decided to sue the world's nuclear heavyweights as "it has a particular awareness of the dire consequences of nuclear weapons," it said in court papers.

Between 1946 and 1958 the United States conducted 67 nuclear tests in the Marshall Islands including Bikini atoll, deBrum told the ICJ's judges.

"The Marshall Islands is on the right side of history... so there's a good chance that they will prevail," said Jacqueline Cabasso of the Western States Legal Foundation, a California-based anti-nuclear organisation.

"We don't know what's going to happen, but it will no doubt put some pressure on those states" to reduce their stockpiles, she added.