In a memorandum, Obama confirmed that India has formally agreed to provide access to the UN nuclear watchdog, the International Atomic Energy Agency (IAEA), of its civilian nuclear reactors.

Indian Prime Minister Manmohan Singh and former US president George W Bush signed the deal in 2008 which allows New Delhi to enter civilian nuclear energy markets for the first time in decades despite its nuclear weapons arsenal.

The US Congress had insisted that India allow international safeguards as a condition of the deal, since India refuses to sign the nuclear Non-Proliferation Treaty.

India gave a list of nuclear reactors to the IAEA, although it said its action is voluntary and does not include military facilities.

Obamas certification clears another hurdle for the agreement to come into force, but more steps remain.

"It shows continued progress and gives reason for optimism that final implementation issues can be resolved soon," said Ted Jones, a director at the US-India Business Council who follows the energy industry.

US businesses have complained that despite the promise of the nuclear agreement, they still cannot seal deals in the growing Indian market without resolution of remaining issues.

For the deal to take effect, the United States and India need to complete an agreement on nuclear fuel reprocessing and New Delhi needs to approve liability protection for US companies.

The nuclear agreement was a milestone in relations between the worlds two largest democracies, which had inconsistent ties during the Cold War when India was non-aligned and sometimes tilted toward the Soviet Union.

Obama has eased early worries in India by showing his commitment to expanding relations. He invited Singh for the first official visit to Washington of his presidency.