The 45-nation Nuclear Suppliers Group, which controls the global flow of civilian atomic exports, is expected to meet Thursday in Vienna on the nuclear deal. Objections by any nation would scuttle the pact.
Japan, the only nation to have suffered atomic attack, had been a major holdout as it pressed for India to sign the nuclear Non-Proliferation Treaty (NPT).
But Chief Cabinet Secretary Nobutaka Machimura said: "It may be biased to view the deal as going against nuclear non-proliferation efforts."
"For example, the issue of global warming has been getting serious in recent years and CO2 emissions from emerging nations such as China and India are becoming a problem," said Machimura, the government's spokesman.
"It is important that India proceeds with nuclear power generation as clean energy."
The deal would give India access to international nuclear technology after being shut out for decades for refusing to sign the NPT.
Anti-nuclear campaigners have petitioned Japan to block the pact, a key priority for Indian Prime Minister Manmohan Singh, who nearly saw the collapse of his government over the issue.
The UN's atomic watchdog, the International Atomic Energy Agency (IAEA), has approved an agreement to inspect some Indian facilities. Singh's former communist allies had argued that inspections would violate Indian sovereignty.
Machimura said the IAEA's supervision of Indian nuclear facilities "can lead to the strengthening of the NPT."
Australia, another country seen as a potential opponent of the India-US nuclear deal, said last week it would not stand in the way.
But the Nuclear Suppliers Group also includes a number of European countries, including in Scandinavia, that are strongly committed to the NPT.
Japan, like the United States, has been seeking a closer alliance with India.
"Japan, which has suffered atomic bomb attacks, should take the initiative in blocking the deal," said Sunao Tsuboi, the 83-year-old chairman of the Hiroshima Council of A-Bomb Sufferers Organisations, a group of bomb survivors.
"It would be a pity if Japan only follows US policy irrespective of what is just," he told AFP. "India needs to join the NPT first if it wants civilian use of nuclear energy."
Japan's main labour union Rengo joined anti-nuclear groups in a letter to Foreign Minister Masahiko Komura urging the government to oppose the India-US deal.
"If this kind of exception is approved, that means that the international community accepts India as a nuclear power. That would shake the nonproliferation regime under the NPT," the letter said.
The Nuclear Suppliers Group is being asked to grant an exception for India, which ordinarily would not be allowed nuclear material as it is not part of the NPT.
India declared itself a nuclear power in 1998 with a nuclear test.
It says the NPT, which came into force in 1970, is unfair by confining nuclear weapons in the hands of five powers -- Britain, China, France, the then Soviet Union and the United States.