Pakistan is violating its "obligations" to the international community by failing to reduce its nuclear arsenal, the Marshall Islands told the UN's highest court on Tuesday.

The small Pacific Island nation is this week launching three unusual cases against India, Pakistan and Britain before the International Court of Justice.

Majuro wants to put a new spotlight on the global nuclear threat, its lawyers said on Monday, by using its own experience with massive US-led nuclear tests in the 1940s and 1950s.

"Pakistan is in breach of its obligations owed to the international community as a whole," when it comes to reducing its nuclear stockpile, said Nicholas Grief, one of the island nation's lawyers.

Islamabad and its nuclear-armed neighbour India "continue to engage in a quantitative build-up and a qualitative improvement" of their atomic stockpiles, added Tony deBrum, a Marshallese government minister.

DeBrum warned that even a "limited nuclear war" involving the two countries would "threaten the existence" of his people.

Pakistan and India have fought three wars since independence from Britain in 1947, two of them over the disputed Himalayan territory of Kashmir.

In 1998, the rival neighbours both demonstrated nuclear weapons capability.

The ICJ's judges are holding hearings for the next week and a half to decide whether it is competent to hear the lawsuits brought against India and Pakistan -- neither of which have signed the 1968 nuclear Non-Proliferation Treaty (NPT).

A third hearing against Britain -- which has signed the NPT -- scheduled to start on Wednesday will be devoted to "preliminary objections" raised by London.

The Marshalls initially sought to bring a case against nine countries it said possessed nuclear arms: Britain, China, France, India, Israel, North Korea, Pakistan, Russia and the United States.

Israel has never admitted to having nuclear weapons.

But the Hague-based ICJ, set up in 1945 to rule in disputes between states, has only admitted three cases against Britain, India and Pakistan, because they have accepted the tribunal's jurisdiction.

Pakistan's lawyers did not attend Tuesday's hearings.

It did however file a counter-claim against Majuro's allegations saying "the court has no jurisdiction to deal with the application" and insisting the case is "not admissible", said ICJ President Ronny Abraham.

- Nuclear 'snow' -

The Marshall Islands' atolls were the scene of extensive nuclear testing in the late 1940s and 1950s which Majuro's lawyers said "vaporised islands and left others uninhabitable for thousands of years."

The so-called "Operation Castle" tests in March and April 1954 were particularly devastating and resulted in massive contamination by nuclear fall-out.

"Scientists estimate that the explosions equalled 1.6 Hiroshima-sized bombs every day for 12 years," said deBrum, who witnessed the explosion of the largest-ever US-built nuclear device called "Castle Bravo" as a nine-year-old boy.

Within a few hours, radioactive fallout started to sprinkle across some atolls in the Marshalls.

"The children thought it was snow. They played in the 'snow'," deBrum said, adding "they even ate the 'snow'."