Italy on Wednesday asked judges in The Hague to order India to release a detained Italian marine, hours before Prime Minister Narendra Modi was due at an EU-India summit in Brussels at which he aims to defuse the long-running row with Rome.

In 2012, India arrested two Italian marines who were escorting an oil tanker on suspicion of killing two fishermen they mistook for pirates. Though they were not charged, the pair were barred from leaving the country.

Massimiliano Latorre was allowed to return home last year for medical treatment, but Salvatore Girone has for four years been confined to Delhi, where he lives at the Italian embassy and reports weekly to Indian police.

The case moved to the Permanent Court of Arbitration in The Hague after India and Italy agreed to suspend all domestic legal proceedings.

Addressing the United Nations tribunal, Francesco Azzarello, Italy's lead lawyer in the case, pledged that Girone would be returned to India to face charges should it bring them once the Hague arbitration is finished.

"The only reason Girone is not allowed to leave India is so that he can act as a de facto guarantee of Italy's obligation to return him for trial," Azzarello said. "A human being cannot be used as a guarantee of the conduct of a state."

Waiting until the end of the case in The Hague, where proceedings are often lengthy, could leave Girone detained without charge for up to eight years, thousands of kilometres from his wife and young children, he said.

India hopes the Brussels summit will bring a thaw in ties with the EU and persuade Italy to refrain from blocking India's membership of a key global group on missile technology. Rome single-handedly scuppered India's bid to join last year.

As part of a broad agenda, the EU plans to raise the issue of the marines with Modi, according to an internal EU council note seen by Reuters.

Italy argues the case of the two marines was beyond the jurisdiction of Indian courts. Marines are viewed by Italy as state officials immune to foreign prosecution. Italy paid $190,000 in compensation to each victim's family.

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