Manmohan Singh's unsually strong words came amid a diplomatic standoff between New Delhi and Rome after the dramatic Italian decision not to send the two to India despite promises made to the Indian Supreme Court.
The prime minister gave his assurance to a delegation of furious Congress MPs from Kerala amid opposition charges of "collusion" between the Italian and Indian governments.
Congress MP P.C. Chacko quoted the prime minister as saying that he would speak to External Affairs Minister Salman Khurshid and "whatever needs to be done through diplomatic channels will be done".
The Supreme Court in February allowed Massimiliano Latorre and Salvatore Girone, accused of killing two poor Indian fishermen off Kerala, to fly to Italy to vote in the Feb 24-25 national election.
Khurshid said earlier that India was studying the Italian decision and that it was a "collective decision" to permit the Italian marines to vote in Italy.
"It was not just a Supreme Court decision but our country's collective decision. They did return after Christmas and we will respond after seeing what reasoning Italy has given," Khurshid said here.
The Italian decision was reportedly taken by its defence and justice ministries in consultation with the country's prime minister's office, the Italian foreign ministry announced Monday.
The marines have been at the centre of a year-long row for shooting the Indian fishermen after reportedly mistaking them for pirates in February 2012.
Italy claims the incident occurred in international waters and has been trying to get Latorre and Girone tried in Italian courts. India argues the killings took place in Indian territory.