The diplomatic crisis over two Italian marines accused of shooting dead two Indian fishermen has taken a new twist, as New Delhi has barred Rome's ambassador from leaving the country.

India's Supreme Court allowed marines Massimiliano Latorre and Salvatore Girone to go home in February, after accusations that they opened fire on a fishing boat off the coast of Kerala.

Latorre and Girone, who both face murder charges, were given licence to vote in Italy's national elections and celebrate Easter, under a written promise from Ambassador Daniele Mancini they would return to stand trial.

Earlier this week, however, Italy announced it would not send back the pair - prompting India's Supreme Court to demand an explanation from Mancini as to why the Rome government has reneged on its promise.

Mancini is expected to file a response by Monday. It is unclear whether he will be allowed to leave India after submitting the explanation.

Mistaken for pirates

Latorre and Girone were part of a security team aboard a cargo ship when they began shooting at a boat they claim they mistook for pirates.

The two were arrested in Kerala and a row on where they should stand trial began.

The Italian government claims the offence took place in international waters, and thus the pair should be tried in Italy under international maritime law. However India maintains it has jurisdiction over the case.

The marines were allowed to travel home for Christmas last December, but were then returned to the Indian authorities.

"Italy has informed the Indian government that, given the formal initiation of an international dispute between the two states, the marines will not return at the end of their home leave," said Italy's foreign minister Giulio Terzi earlier this week.

"Italy always maintained that the Indian authorities' behaviour violates international law, in particular, it violates the right of immunity from the foreign country's judicial system."

India's Prime Minister Manmohan Singh replied saying Italy's decision was "unacceptable".

"They violate every rule of diplomatic discourse and call into question solemn commitments given by the accredited representative of a sovereign government to our Supreme Court," Singh told Parliament. "There will be consequences."