Nato has warned India that using anti-terrorism legislation against two Italian marines accused of killing two fishermen would undermine international efforts to combat piracy.
Massimiliano Latorre and Salvatore Girone, part of a military security team protecting the tanker Enrica Lexie, stand accused of shooting the fishermen after mistaking them for pirates off the southern Indian state of Kerala in February 2012.
"I am very concerned about the situation of the two Italian sailors. I am also concerned by the suggestion that they could be tried for terrorism offences," Nato chief Anders Fogh Rasmussen said.
"This could have possible negative implications for the international fight against piracy, a fight which is in all of our interest."
Italian prime minister Enrico Letta said the proposal was unacceptable. "Italy and the European Union will react," he warned.
Letta tweeted that the Italian government "totally rejects the use of the concept of terrorism".
Latorre and Girone have been living in the Italian embassy in New Delhi since the shootings. They were allowed to return home to vote in the February 2013 elections.
Rome initially refused to return them, arguing they should be tried in Italy because the incident took place in international waters.
India's media said that Rome's decision not to send back the two Italian sailors was an embarrassment for the Indian government.
The Times of India said it "required an appalling degree of naiveté on the part of Indian institutions to let the marines go in the fond belief that the Italians would send them back".
India summoned the Italian ambassador in Delhi and said that Italy's action was "unacceptable".
The two marines eventually flew back to Delhi in March 2013.
India's attorney general has asked that they would be tried under the anti-piracy and anti-terrorism law but that the death penalty available under that legislation would not be imposed.
The Indian Supreme Court is due to hold a hearing on 18 February to decide whether to validate or reject the attorney general's request.