NEW DELHI (Reuters) - India's half-century-old decision to end a territorial dispute with Sri Lanka over a tiny island has become a hot-button election issue, with Prime Minister Narendra Modi's party accusing the rival Congress of compromising fishermen's rights.

       A 1976 agreement barred Indian fishermen from waters around the 285-acre (115-hectare) island in the Palk Strait that divides the neighbours, two years after a pact on maritime boundaries gave Colombo rights over it.

       On Monday, Foreign Minister Subrahmanyam Jaishankar said Sri Lanka had detained more than 6,000 Indian fishermen and 1,175 fishing vessels over the last 20 years, following the 1976 deal on the island, located 33 km (21 miles) off India's coast.

       His comments come a day after Modi accused the Congress of having "callously" given away the island, called Katchatheevu.

       "Weakening India’s unity, integrity and interests has been Congress’ way of working for 75 years and counting," Modi said on social media platform X.

       In response, Congress chief Mallikarjun Kharge said the 1974 agreement had been "based on a friendly gesture" and suggested that Modi's comments came with an eye to general elections set to start on April 19, at which he will seek a rare third term.

       Kharge said Modi raised the sensitive issue on the eve of the elections, though his government's attorney general had told the Supreme Court in 2014 that India would "have to go to war" if it wanted to recover the island from Sri Lanka.

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       The office of Sri Lanka's president and its foreign ministry did not immediately respond to requests for comment.

       Discontent over the curtailed fishing rights has grown in the southern coastal state of Tamil Nadu neighbouring Sri Lanka, leading to two legal challenges to the agreements during the last two decades that are still pending in the Supreme Court.

       Tamil Nadu goes to the polls on April 19, the first phase of seven rounds of voting set to end on June 1.

       Modi's Bharatiya Janata Party is trying to make inroads in the state, where it won none of the state's 39 seats in India's 545-member parliament during the last election.

       Jaishankar did not comment if the government would seek to alter the status of the island, as the matter was in the Supreme Court.

       (Reporting by Krishn Kaushik; Additional reporting by Uditha Jayasinghe in Colombo; Editing by Clarence Fernandez)