ISLAMABAD, Pakistan — The Bush administration urged Pakistan not to expand its nuclear weapons program yesterday after an American think tank said Islamabad was building a reactor that could generate plutonium for up to 50 atomic bombs a year.

The Foreign Ministry did not deny the report by the Washington-based Institute for Science and International Security, which said Pakistan was expanding its atomic arms capabilities.

“We have been aware of these plans and we discourage any use of that facility for military purposes, such as weapons development,” the White House spokesman, Tony Snow, said.

He noted that Pakistan has not signed the Nuclear Nonproliferation Treaty. Pakistan’s Foreign Ministry said yesterday that it opposes a regional nuclear arms race.

Asked if Washington had sought Pakistani assurances that it wouldn’t use the new reactor to produce plutonium for nuclear weapons, Mr. Snow said, “Not that I’m aware of.”

The institute report said satellite photos of Pakistan’s Khushab atomic site, about 105 miles southwest of the capital, Islamabad, showed what appeared to be construction of a reactor capable of producing enough plutonium for 40 to 50 nuclear weapons a year.

The move could signal an acceleration of regional nuclear proliferation and the new reactor could be finished within a few years, the report said.

“South Asia may be heading for a nuclear arms race that could lead to arsenals growing into the hundreds of nuclear weapons, or at a minimum vastly expanded stockpiles of military fissile material,” the report, which was first described by the Washington Post, said.

It concluded that the expansion plans would represent a 20-fold increase of Pakistan’s existing capabilities.

A spokeswoman for Pakistan’s Foreign Ministry, Tasnim Aslam, did not directly answer questions on the report.

“This ought to be no revelation to anyone because Pakistan is a nuclear weapon state.” Ms. Aslam said.”I have no specific comments on Pakistan’s facilities.”

Ms. Aslam said Pakistan was not the first country to introduce nuclear weapons into South Asia. Pakistan conducted its only nuclear tests in May 1998 after Indian nuclear tests earlier that month. India detonated its first nuclear explosion in 1974.

“We were not the first to test nuclear weapons in this region and that remains our position,” Ms. Aslam said during a news conference. “We do not want an arms race in this region.”

Nuclear-armed neighbors Pakistan and India came close to open conflict in 2002 after terrorists attacked India’s parliament. New Delhi accused Islamabad-backed militants of carrying out the attack, but Pakistan denied the claims. Both countries have since embarked on a stop-start peace process.

In February 2004, Abdul Qadeer Khan, considered to be the father of Pakistan’s atomic program, confessed to giving nuclear technology to Iran, North Korea, and Libya.

President Musharraf has pardoned Mr. Khan.

A pending nuclear cooperation agreement between the Bush administration and India would give New Delhi access to sensitive American nuclear technology in exchange for agreeing to more stringent safeguards over its civilian nuclear reactors.

Pakistan has criticized the deal, which Congress must still approve, as one-sided, and demanded similar access to American atomic technology.