New Delhi, March 12 - Indian Premier Manmohan Singh on Tuesday described as ''absolutely unacceptable'' Italy's decision not to send back two marines for trial on charges of homicide relating to the alleged killing of two Indian fishermen during an anti-piracy mission in February 2012. He also said he would ask Foreign Minister Salman Khurshid to take up the matter with Italy. On Monday the Italian government announced that Massimiliano Latorre and Salvatore Girone would not be returning to face trial in India after the supreme court in New Delhi granted them a four-week permit to exercise their right to vote in last month's general elections. They were also allowed back for the Christmas holidays and returned to India once their leave expired. They had pledged to do so in this case as well. However in a statement Monday, the foreign ministry said that ''the conduct of Indian authorities violated international rights'' by detaining them for over a year. It also said it was open to allowing an international arbitrator assess the case. The chief minister of Kerala, the southern state off which the incident occurred, also described Italy's decision as ''unacceptable''. Meanwhile on Tuesday local fishermen burned images of the Indian Premier and one of the marines in protest over the decision.