(NEW DELHI) — India‘s prime minister demanded Wednesday that Italy return two marines accused of killing a pair of fishermen last year or face unspecified consequences, deepening a diplomatic dispute between the two countries.

The Indian government allowed the marines to return home in February to vote in national elections and to celebrate Easter with a promise from the Italian ambassador that they would return to stand trial.Italy announced Monday it would not send them back.

The dispute, along with allegations of bribes and kickbacks in a $750 million helicopter deal between India’s military and an Italian defense company, has badly damaged relations between the two nations. Angry politicians have attacked the government for allowing the two Italians to leave the country in the first place, and they demanded retaliation.

“I want to make it clear that these actions of the government of Italy are not acceptable,” Prime Minister Manmohan Singh told Parliament on Wednesday. “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 said Italy’s actions were damaging bilateral relations that were built on trust.

“Our government has insisted that the Italian authorities respect the undertaking they had given to the Supreme Court and return the two accused persons to stand trial in India,” he said. If they don’t, “there will be consequences.”

He did not elaborate on what those would be.

The marines, Massimilian Latorre and Salvatore Girone, were part of a military security team aboard a cargo ship when they opened fire on a fishing boat in February last year that they said they mistook for a pirate craft, killing the two fishermen. Italy maintains the shooting occurred in international waters and that Rome should have jurisdiction. India says the ship was in Indian territorial waters.

One former official said India should follow a calibrated diplomatic strategy to deal with the dispute.

“India should make a strong representation to the Italian government that this is a breach of agreement and contempt of India’s judicial system,” said Lalit Mansingh, a former foreign secretary.

He said India should also stop receiving Italian delegations, including trade groups, shutting out the attractive Indian market to Italian companies.

“The message can be conveyed through diplomatic channels, and also informally, that in view of this kind of behavior, India would consider debarring Italian companies from taking part in government tenders,” Mansingh said.

“Today all European countries are looking at India as a potential market and a place for investment. If the Italians foolishly want to get out of this market, then so be it. The loss will be theirs,” he said.

Also Wednesday, Oommen Chandy, the top elected official from the southern state of Kerala, which is near the site of the shooting, met with Singh to urge him to press Italy for the return of the marines and ensure justice for the families of the fishermen.

The wife of one of the slain fishermen said the decision to let the marines go home to vote appeared to be part of a plan to spare them from facing trial.

“This is nothing but a conspiracy at the highest level,” said the wife, Dora Valantine. “The Indian government should ensure that they bring back the two and make them stand trial in this country.”

Last year, Italy paid compensation of 10 million rupees ($192,308) each to the families of the fishermen.

The Indian government had earlier allowed the marines to go home over the Christmas holidays, after which they returned to India. In February, India’s Supreme Court allowed them to return home to vote after the Italian ambassador gave a guarantee they would return.

The marines said they had put their faith in the Italian government to help them out.

“I knew that our government wasn’t abandoning us. It wouldn’t abandon us. They gave us four weeks from when we returned to Italy to vote, and I felt that something would happen, something positive, I mean,” Girone was quoted as saying by the Milan daily Corriere della Sera.

But he said they were not celebrating their release.

“There isn’t anything to celebrate. Our case is not over yet,” Girone said.

The Italian government came under attack in that country’s media on Tuesday, with the Rome daily La Repubblica saying the move by Premier Mario Monti‘s caretaker government “dealt a terrible blow to our credibility from the point of view of international image.”