"We are off the bilateral level, to raise the dispute to an international level: we are still willing to talk to the Indians", but "we have no other option than resorting to international arbitration", she told the Senate. (Agencies)
Rome does not consider the court proceedings in India "valid", Mogherini was quoted as saying by a news agency.
"We do not accept a (future) Indian trial whose validity we do not recognise," she said.
"We are mapping out a panel of experts under the leadership of a coordinator to pursue the new phase," Mogherini said.
Marines Massimiliano Latorre and Salvatore Girone are awaiting trial over the killing of two Indian fishermen off Kerala coast in February 2012, sparking diplomatic tensions between India and Italy.
The marines, deployed on the Italian-flagged oil tanker MT Enrica Lexie, had contended they mistook the fishermen for pirates. They are now staying in the Italian Embassy in New Delhi awaiting trial.
Italy has requested Latorre and Girone be allowed to return home and the whole case be dropped.
Rome wants the marines to be tried in Italy, claiming the incident took place in international waters. However, New Delhi says it has the right to try the Italians as the victims were Indians on board an Indian fishing boat.
A special court on March 31 has fixed July 31 for hearing the case of the two Italian marines.
The Supreme Court had sought response from the Centre on the petition filed by the two marines challenging the jurisdiction of National Investigation Agency (NIA) to prosecute and probe the case.
The marines had moved the Apex Court more than a month after the Indian Government had decided to withdraw their prosecution under the stringent anti-piracy SUA law that attracts death penalty as maximum punishment.
The Centre on February 24 had told the apex court that the marines would not be prosecuted under the Suppression of Unlawful Acts Against Safety of Maritime Navigation And Fixed Platforms on Continental Shelf Act (SUA).
"We are off the bilateral level, to raise the dispute to an international level: we are still willing to talk to the Indians", but "we have no other option than resorting to international arbitration", she told the Senate.