* Italian marines say thought fishermen were pirates
* Arrest and detention straining Rome-Delhi ties
* Italy says marines' arrest undermines piracy fight
ROME, March 13 (Reuters) - Italy on Tuesday accused Indian authorities of using trickery to arrest two Italian marines accused of killing two Indian fishermen they thought were pirates, and said Delhi's attitude risked undermining the international fight against piracy.
Foreign Minister Giulio Terzi told the Senate that Italy was pursuing every possible solution to win the marines' release almost a month after the incident.
The marines, Massimiliano Latorre and Salvatore Girone, were stationed on a merchant ship off the southern Indian coast to protect it from pirate attacks. The marines said they shot at the fishing boat on Feb. 15 believing it to be a pirate vessel.
Terzi said their merchant ship, the Enrica Lexie, entered the Indian port of Kochi on Feb. 20 after authorities asked the crew to help identify some suspected pirates. The marines were then arrested by some 30 armed Indian officials in what the foreign minister said was "a subterfuge by the local police".
Tensions between the two countries have been running high ever since, with India turning a deaf ear to Italy's insistence that jurisdiction over the marines should lie with them because the incident occurred in international waters.
Terzi said Italy was backed by "important countries" who feared India was endangering the anti-piracy battle.
"The precedent set by India can have grave consequences on the effectiveness of international operations against piracy and terrorism," he said.
The European Union said last week it would intervene to seek a resolution and on Tuesday Prime Minister Mario Monti met with EU foreign affairs chief Catherine Ashton to discuss the matter. (Reporting by Roberto Landucci; Writing by Gavin Jones; Editing by Ben Harding)