Indian opposition leader Narendra Modi launched a direct attack on rival Sonia Gandhi today, suggesting she had sought to shield Italian marines accused of killing two Indian fishermen in a case that has strained ties with Italy.

Gandhi is Italian-born and Modi's Hindu nationalist party has in the past questioned her credentials to lead the country. Such attacks have since ceased and the powerful head of the Congress party is accepted as an Indian.

But in a campaign speech ahead of next week's election, Modi appeared to rake up her foreign origins again as he questioned the government's handling of the 2012 case involving the Italian marines who are facing trial in India. He criticized the government for letting the two men go home during Christmas in 2012 and said they returned to India only after the Supreme Court cracked the whip.

"Who was it in Delhi who allowed them to flee? On whose direction was the government acting?" Modi asked at a campaign rally in the remote northeastern state of Arunachal Pradesh. He spoke a day after Gandhi accused the opposition of pursuing a divisive ideology in a nation rich in diversity.

"Will Sonia Gandhi teach us patriotism? I urge you madam, never question the patriotism of the 1.5 billion Indians in the country," Modi said. "Italian soldiers kill our fishermen, they are allowed to go. Is that defending the country's interest? And you are trying to give us a certificate in patriotism."

There was no immediate comment from the Congress which is fighting a tough election under the leadership of Sonia's son and the fourth generation member of the Nehru-Gandhi dynasty, Rahul Gandhi.

India has vowed to put the two Italians on trial but has dropped a plan to prosecute them under a tough new anti-piracy law. The sailors, part of a military security team protecting a privately owned cargo ship, say they mistook the fishermen for pirates and fired warning shots into the water during the incident in February 2012, off the coast of Kerala state.

Modi demanded the government come clear on the whereabouts of the sailors, who are thought to be at the Italian embassy as they await trial. "Which prison are they in? Who is responsible?" he said. Modi, a polarising figure, has throughout the campaign trained his guns on India's most powerful family, attacking Rahul as a privileged "shehzada," or prince.

Opinion polls suggest Modi's Bharatiya Janata Party-led alliance is increasing the lead over the ruling Congress party and its allies and is best placed to form a coalition government after the elections, set for April and May. The BJP-led coalition could win 233 seats out of the 543 at stake, within striking distance of the 272 required to rule, a poll by ABP News-Nielsen said at the weekend. The ruling alliance could slip to 119 seats, it said. Modi's personal ratings remained the highest, with 54 percent of those polled in March picking him for prime minister. The rating was down from 57 percent in February, however.

"You gave them 60 years, give me 60 months. I promise I will change the country," Modi told today's rally.