The government appears to have been taken aback by Italy's decision not to send back its two marines accused of killing Indian fishermen with external affairs Salman Khurshid merely saying that the government was studying Rome's decision.

It was a "collective decision" to send back the two Italian marines being tried for murder in India to vote in that country's general election, Khurshid said on Tuesday.

"It was not just Supreme Court decision but our country's collective decision. They did return back after Christmas and we will respond after seeing what reasoning Italy has given," Khurshid told reporters.

The decision not to send Massimiliano Latorre and Salvatore Girone, who had been allowed to come home to vote and meet their families, back to India was taken by Italy's defence and justice ministries in consultation with the country's prime minister's office.

The two marines have been at the centre of a year-long international row for allegedly shooting dead two Indian fishermen after mistaking them for pirates off the southern Indian state of Kerala in February 2012.

Italy claims the incident occurred in international waters and has been trying to get Latorre and Girone tried in Italian courts, while India contends the shooting occurred in its own territory.

Supreme Court in February allowed Latorre and Girone to return to Italy to vote in the February 24-25 national election. It had earlier allowed them to go home for Christmas.