The United States told the UN's top court Wednesday it had "a duty" not to take a position on a bitter dispute over the British-ruled Chagos islands, home to a strategic joint US military base.
Judges at the International Court of Justice are listening this week to arguments from various countries in a case brought by the United Nations over the future of the Indian Ocean archipelago, which is claimed by Mauritius.
London split off the remote islands from Mauritius in 1965, three years before Port Louis gained independence. The archipelago's status has been at the centre of a bitter dispute spanning five decades since.
US representative Jennifer Newstead said even though the UN request was for a "non-binding advisory opinion" on the row, the ICJ's 15 judges were in fact being asked to rule in a bilateral territorial dispute.
"This places the court in an untenable position," said Newstead, a State Department legal advisor, because it asked the ICJ to rule in a sovereignty dispute, when it was only meant to give a legal opinion.
Therefore "the court has a duty to decline to provide the opinion," Newstead said.
In a diplomatic blow to Britain, the UN General Assembly last year adopted a resolution presented by Mauritius and backed by African countries asking the Hague-based ICJ to offer legal advice on the island chain's fate.
The judges are asked to give an opinion whether the "process of decolonisation of Mauritius was lawfully completed" after Chagos was split off.
They are also asked to give their view on the consequences of Britain's continued administration of the islands -- including the inability of thousands of Chagossians who were evicted in the 1970s, to return to their homes.
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The African Union and 22 countries -- which also includes the US, Germany and several Asian and Latin American nations -- are making statements during the four-day hearing.
After the hearings, the ICJ is expected to hand down a non-binding advisory opinion, but the judges' ruling may take several months or even years.
However, an opinion still carries weight and a finding in favour of Mauritius may strengthen its hand in future negotiations. It could even lay the foundation for an eventual formal claim before the ICJ -- set up in 1946 to rule in disputes between countries.
Mauritius, which declared independence in 1968 argues that it was illegal for London to break up its territory while still under colonial rule.
Afterwards London established a combined military base with the US on Diego Garcia, the largest of the islands.
Britain said it would give back the islands to Mauritius "when no longer required for defence purposes."
The Diego Garcia base still plays a key strategic role in US military operations, including serving as a staging ground for US bombing campaigns in Afghanistan and Iraq.