Barack Obama on Sunday began an unprecedented second visit to India by a serving US president, aiming to consolidate what he has called one of the "defining partnerships of the 21st century".

Obama will be the first US president to be chief guest at India's Republic Day parade, acolourfulcelebration of the country's military might and cultural diversity, signalling a growing closeness between the world's two largest democracies.

He received a hug from Prime Minister Narendra Modi as he disembarked from Air Force One in New Delhi at the start of a visit, which is seen assymbolisinga new warmth in sometimes strained bilateral ties.

Modi has invited Obama to co-host a radio phone-in show and is reportedly planning to host a private dinner at his Delhi home for the president, who will be accompanied by First Lady Michelle Obama.

The couple was scheduledto visitedthe Taj Mahal, India's famed monument to love, but their trip has been cut short to allow Obama to travel on to Saudi Arabia and pay respects to new King Salman.

Modi was a Washington outcast only a year ago, and Obama's three-day visit caps a remarkable turnaround in relations between the two countries after a diplomatic row in late 2013 that saw the Stars and Stripes torched on the streets of Delhi.

In an interview with the India Today magazine ahead of his arrival, Obama said the two countries were "natural partners".

"When I addressed the Indian Parliament on my last visit (in 2010), I outlined my vision for how we could become global partners meeting global challenges," he said.

"I'd like to think that the stars are aligned to finallyrealisethe vision I outlined."

Both sides share a common goal in wanting acounter-balanceto China, with Modi seen as taking a more assertive line on India's powerfulneighbourthan the previous regime.

Obama's trip comes just months after Modi's first official visit to the United States, and with so little time to lay the groundwork it is expected to be stronger on symbolism than on content.

Nonetheless, a deal on intelligence sharing is on the cards and decade-olddefencecooperation pact is expected to be upgraded.



'Willing to do business'


Climate change -- a key priority for Obama -- and Afghanistan are expected to be discussed, while the two leaders will make a joint address to company bosses on Monday.

Indian broadcaster NDTV said Modi and Obama would discuss a possible breakthrough on a stalled nuclear deal when they go for a walk on Sunday.

The two countries signed the landmark deal giving India access to civilian nuclear technology in 2008, but it has been held up by US concerns over India's strict laws on liability.

India has offered to set up an insurance pool to indemnify companies that build reactors in the country against liability in case of a nuclear accident.

Modi's election in May 2014 was a potential headache for the US, which had blacklisted the Hindu nationalist for more than a decade after deadly communal riots in Gujarat when he was the state's chief minister.

He was only brought in from the cold last February when the US ambassador travelled to Gujarat once it appeared Modi was likely to end thecentre-left Congress party's 10-year rule.

The transformation since has been spectacular, with both men heartened by their meeting ofmindson a range of issues in Washington in September.

"I think Modi surprised everyone by, with very little hesitation, embracing the United States," said Milan Vaishnav of the Carnegie Endowment for International Peace.

"To give credit where credit is due, the Obama Administration stepped in very quickly after his election to signal that it was willing to do business".

The Indian capital is expected to be under virtual lockdown for the January 26 celebrations.

Obama will watch the parade -- a huge spectacle featuring everything from tanks to camels and tribal dancers -- from behind abullet-proofglass enclosure with Modi.

Beggars have been cleared from many streets and traders in Connaught Place, the city's central commercial hub, said they had been told to shut down during the US president's visit.

"They won't allow me here on the 26th," said 16-year-old Amit Kumar, who shines shoes on the pavement. "They don't want any unwanted people on the streets."