Mohammed ElBaradei, the head of the International Atomic Energy Authority, said the accord would end India's nuclear isolation and spur global non-proliferation efforts.
Jacques Chirac, the French president, said it would boost efforts to combat both climate change and the spread of nuclear weapons.
Under the deal, the US will offer India nuclear fuel and technology provided it separates its civil and military nuclear facilities and places the former under international inspections.
Some US lawmakers and nuclear experts have criticised the pact, saying it weakens international safeguards, especially the nuclear Non-Proliferation Treaty (NPT), which India has refused to sign, calling it discriminatory.
The NPT grants China, the US, Russia, France and Britain status as nuclear weapons states, but bars other signatory countries from having such weapons.
ElBaradei: Deal timely for
efforts to check nuclear safety
But the support of ElBaradei gives the deal an important seal of approval. He said the deal would help satisfy India's growing energy needs.
"It would also bring India closer as an important partner in the non-proliferation regime," he said. "It would be a milestone, timely for ongoing efforts to consolidate the non-proliferation regime, combat nuclear terrorism and strengthen nuclear safety."
'A responsible power'
France and India also signed an agreement on civilian nuclear co-operation during Chirac's visit there last month, where he praised the merits of French nuclear power technology.
Chirac called India "a responsible power". Ensuring that India has access to civilian nuclear technology is "indispensable" in enabling it to meet "its immense energy needs, while limiting its emissions of greenhouse gases", the French leader said.
The deal still needs to be approved by the US Congress, where it is sure to come under close scrutiny. Tom Lantos, the ranking Democrat on the House International Relations Committee, gave it a cautious welcome.
"A reliable and dependable strategic partnership is in the interest of both our great countries, and this agreement could herald an even closer relationship between the United States and India," he said.
"Given the unprecedented nature of this agreement, the Congress will have to carefully examine the details of the separation plan to assure ourselves and our international partners that this agreement will indeed support our shared political and security objectives."
In New Delhi, Nicholas Burns, the US undersecretary of state who negotiated the pact, said it was "unique" to India and would not be repeated with other countries such as neighbour and rival Pakistan.
The US expects international
support for the deal
"What distinguishes India is that India has protected its nuclear technology over the 30 years of the Indian nuclear programme. India has not proliferated, unlike North Korea, which has been a major proliferator," Burns said.
"India has brought itself into conformance with all the international guidelines pertaining to nuclear technology, unlike Iran for instance, which has been a great violator of those international programmes," he said.
"We have always seen this deal to be a unique deal for India alone. We certainly would never consider entertaining this with a country that had proliferated, for instance North Korea, or a country that had lied to the IAEA - Iran."
The US has explained to Pakistan that a similar agreement would not be made with Islamabad, Burns said.
Pakistan has had "proliferation problems of a quite serious nature over the last several years that would make this kind of deal impossible, and we've been very up front and direct with the Pakistanis in saying that", he added.
Billions of dollars
Burns said Russia and European allies had already indicated that they would support the deal, which will be worth billions of dollars to US firms that provide nuclear technology to India.
"So we do expect broad scale international support," he said.
The US has agreed to allow Western firms to invest in India's nuclear technology, which will require a change in US law. Condoleezza Rice, the US secretary of state, was to start making phone calls to lawmakers later on Thursday, Burns said.