Barack Obama and Narendra Modi vowed co-operation between the United States and India on nuclear power and climate change during a meeting at the White House Tuesday. Offering Modi a warm embrace, Obama said it was his "great pleasure to welcome back my friend Prime Minister Modi to the Oval Office."

Obama also offered high profile backing for Modi's bid to normalise India's once clandestine nuclear program. A series of nuclear tests from the 1970s saw India hit with expulsion from a nuclear trading group and with UN sanctions.

In 2005, then-president George W. Bush lifted a three decade long moratorium on nuclear co-operation with India. Obama has sought to further remove the nuclear issue as an obstacle to better ties between two of the world's largest democracies.

"We discussed the progress that we have made around civil nuclear energy and I indicated our support for India becoming part of the nuclear suppliers group," Obama said. Even with US backing that move is likely to be opposed by China.

Beijing sees India warily and closer India-US relations as a potential check on its regional aspirations. But Obama's overture was another symbolic marker on India - and Modi's - long walk in from the cold.

When Obama took office, the Hindu nationalist leader was still banned from visiting the United States for his role in anti-Muslim riots that killed hundreds. Obama leaves office with Modi transformed from persona non grata to celebrated guest. His visa ban was overturned after a sweeping election victory in 2014.

But even between Delhi and Washington turning warm words into concrete agreement has proven difficult. A proposed bilateral investment treaty has languished for years, as New Delhi has taken a tough negotiating line.

Modi, who faces reelection in 2019, has pegged his political future on a reform agenda and boosting the economy. He has also made nuclear energy a priority, to offset horrendous levels of air pollution.