US President George W. Bush and Indian Prime Minister Manmohan Singh signed the deal in New Delhi on Thursday that paves the way for the lifting of three-decade-old restrictions on sharing civilian nuclear technology.

Bush and Singh hailed the deal as historic, with the arrangement being seen as a landmark moment in bilateral ties that have steadily improved following decades of strained relations that date back to the Cold War.

China voiced the strongest official note of regional caution shortly after the deal was announced when it said New Delhi and Washington must follow global nuclear non-proliferation rules.

"Cooperation must conform with the requirements and provisions of the international non-proliferation regime and the obligations undertaken by all countries," foreign ministry spokesman Qin Gang said on Thursday.

India and similarly nuclear-armed regional rival Pakistan have refused to sign on to the Nuclear Non-Proliferation Treaty.

However China, whose containment was believed to be one reason for the United States wanting to embrace India on the nuclear issue, publicly refrained from saying anything more critical.

Key regional US ally Japan, which is also seeking closer relations with India, welcomed the deal and rejected assertions that New Delhi should be held to the same standards as international pariah North Korea.

"India is a country that shares the values of freedom, democracy, basic human rights and the rule of law with the United States and Japan," Chief Cabinet Secretary and government spokesman Shinzo Abe told reporters.

"It is wrong to discuss the Indian nuclear issue and that of North Korea on the same level."

Japan and the United States, along with China, South Korea and Russia, are involved in long-running diplomatic efforts to convince Pyongyang to abandon its nuclear weapons programme.

Like India and Pakistan, North Korea is not a signatory to the Nuclear Non-Proliferation Treaty.