US President Barack Obama said on Tuesday that the United States could be India's "best partner", but urged his hosts to do more in the battle against climate change.

Speaking to an audience of mainly young people as he wrapped up a three-day visit to New Delhi, Obama said the two countries could forge "one of the defining partnerships of this century", but warned the war against climate change would not "stand a chance" without India.

The speech was the finale of a visit that has seen a dramatic upturn in an often-troubled relationship, including the signing of a new "friendship" pact between Obama and Prime Minister Narendra Modi.

The right-wing prime minister was a pariah in Washington less than a year ago, but has developed a close bond with Obama.

"India and the United States are not just natural partners - I believe America can be India's best partner," said Obama after receiving a rapturous welcome from a group of around 1,500 people.

"Of course, only Indians can decide India's role in the world. But I'm here because I am absolutely convinced that both our peoples will have more jobs and opportunity, our nations will be more secure, and the world will be a safer and more just place when our two democracies stand together."

The speech also contained a plea for people to respect one another's religion in India, where the election of Hindu nationalist Modi has given rise to fears among the country's large Muslim minority.

"India will succeed as long as it's not splintered along religious lines," Obama said.

Obama stressed the need for both the government and every citizen to uphold religious freedom, considered to be one of the controversial topics in India.

"In both our countries, in all countries, upholding this fundamental freedom is the responsibility of government, but it's also the responsibility of every person," he said.

Both Obama and Modi have been at pains to demonstrate their personal rapport during the US president's visit, and announced a breakthrough on a long-stalled nuclear power deal on Sunday.

Obama was also the chief guest at Monday's Republic Day parade - one of the biggest honors that India can bestow on a foreign leader.

The US is looking to reinvigorate alliances in the Asia-Pacific as part of Obama's "pivot" east.

Speaking after their talks on Sunday, Modi said he would not be pressured on climate by any country - comments seen in part as aimed at China after it agreed to new carbon emissions targets with the US.

But Obama warned the battle against climate change was doomed unless developing countries reduce dependence on fossil fuels.

"But here's the truth: Even if countries like the United States curb our emissions, if growing countries like India - with soaring energy needs - don't also embrace cleaner fuels, then we don't stand a chance against climate change", he said.

India has balked at committing itself to major cuts in carbon emissions ahead of a December climate summit.

AFP - Xinhua