New Delhi ? A landmark nuclear pact reached Thursday by President Bush and Indian Prime Minister Manmohan Singh faced tough scrutiny by Congress and international regulators amid concerns that it would allow India to expand its nuclear arsenal by hundreds of weapons.

If approved by Congress, the accord would recognize India as a nuclear military power and herald a major expansion in ties between the world’s largest democracy and the United States after decades of strained relations.

“What this agreement says is things change. Times change,” Bush said, appearing with Singh at Hyderabad House, the Indian government’s guest residence.

“We have made history today, and I thank you,” Singh said.

Some U.S. lawmakers and many arms control experts said the pact would undercut the global system designed to halt the spread of nuclear arms, making it harder to rein in suspected Iranian and North Korean nuclear weapons programs.

Rep. Edward Markey, D-Mass., a co-sponsor of bipartisan legislation that would block the deal, said it made a mockery of the cornerstone of the system, the Nuclear Non-Proliferation Treaty, which India never signed.

“With one simple move the president has blown a hole in the nuclear rules that the entire world has been playing by,” Markey said. “It empowers the hawks in every rogue nation to put their nuclear weapons plans on steroids.”

While welcoming the accord, several senior Republican lawmakers, including Rep. Henry Hyde, R-Ill., chairman of the House International Relations Committee, said they would give it a rigorous review.

U.S. President George W. Bush, right, and Indian Prime Minister Manmohan Singh make comments to reporters after an arrival ceremony at Rashtrapati Bhavan, or the President's House, in New Delhi, India, Thursday, March 2, 2006.

The accord also will have to pass muster with the 45-nation Nuclear Suppliers Group. It was formed to regulate civilian nuclear trade after India detonated a device in 1974 using plutonium obtained from a Canadian-made civilian reactor that was supplied with heavy water by the United States.

The group decides matters by consensus. Several members expressed misgivings with the India-U.S. agreement.

The accord would end India’s status as a nuclear renegade and clear the way for U.S. companies to sell civilian nuclear equipment to India. In return, India would declare 14 of 22 reactors part of its civilian program and place them under international monitoring.

Bush said Americans would benefit because increased use of nuclear power in India would reduce global demand for oil. With 1.1 billion people and a fast-growing economy, India is consuming a larger share of global energy supplies.