Killer Marines: Furious India rejects Italian verdict
Singh conveyed this to delegations of Kerala MPs from Left and Congress, who met him separately to express anguish and demand his intervention in the "serious" matter.
The Prime Minister said it was "unacceptable", according to CPI (M) MPs K N Balagopal and M B Rajesh.
Singh told the Left MPs that he will ask External Affairs Minister Salman Khurshid to take up the issue with Italy.
His comments came in the wake of Italian Foreign Ministry statement that the marines Massimiliano Latorre and Salvatore Girone will not return to India from Italy where they had gone to cast vote in last month's elections after special permission granted by the Supreme Court.
The two marines are facing trial for allegedly killing two fishermen off the Kerala coast in February last year, mistaking them for pirates.
The Italian Ministry claimed India had not responded to its requests to seek a diplomatic solution to the case and there was now a formal dispute between the two countries over the terms of the United Nations Convention of the Law of the Sea.
Khurshid said the government is studying Italy's decision, its reasons as also implications.