WHEN the United States revealed last October that Pakistan had supplied uranium enrichment technology to North Korea, the Bush administration did not pin the blame on the U.S.-backed military ruler in Islamabad, General Pervez Musharraf. Secretary of State Colin Powell endorsed Musharraf's claim that he had stopped the nuclear transfers to Pyongyang initiated by his civilian predecessors.

Evidence now suggests that nuclear collaboration between Pyongyang and Islamabad did not stop when Musharraf staged his army coup in October 1999, and may still be continuing.

Firm U.S. action is urgently needed to guard against further Pakistani nuclear transfers not only to North Korea but also to other would-be nuclear powers, notably Saudi Arabia, and to prevent the leakage of Pakistani fissile material to terrorist groups.

After a six-month internal policy battle, the administration decided in late March not to impose sanctions against Pakistan for helping the North Korean nuclear program. Yet evidence abounds that Pakistan provided uranium enrichment technology to Pyongyang beginning in 1998 in exchange for missiles.

A Department of Energy report in 1999 and a CIA report in June 2001, recently revealed by investigative reporter Seymour Hersh, provide detailed evidence of this collaboration and of Musharraf's complicity in continuing nuclear transfers to Pyongyang after his coup.

Soon after Musharraf took over in 1999, the United States pressed him to remove the controversial czar of the Pakistani nuclear program, A.Q. Khan, who has visited North Korea 13 times. Musharraf did not oust Khan until March 2001 and continued to retain him as a special adviser.

Three months later, the administration was still openly suspicious of Pakistani collaboration with Pyongyang. Deputy Secretary of State Richard Armitage told The Financial Times on June 1, 2001, that ``people who were employed by the nuclear agency and have retired may be assisting North Korea with its nuclear program.''

The ``smoking gun'' that triggered the U.S. confrontation with North Korea over the uranium issue last October was the discovery of an incriminating document two months earlier showing that Pakistan was still helping North Korea at that late date, three years after Musharraf took power.

According to the CIA, Pakistan used U.S.-supplied C-130 transport planes to ship six Nodong missiles from North Korea to the A.Q. Khan Research Laboratories in March. This provoked the imposition of trade sanctions against the Khan Laboratories and the North Korean Changgwong Corporation, barring them from trade with U.S. firms. But the State Department stressed that the sanctions related solely to missiles and had nothing to do with nuclear technology.

The continuing transfer of North Korean missiles to Pakistan raises the question of how Islamabad is paying for them. It was Pakistan's inability to pay in cash in 1998 that prompted its offer to pay instead with uranium enrichment technology. In this latest transaction, if Islamabad is not paying with nuclear technology, is it using some of the cash given by the United States since 9/11 to buy missiles from Pyongyang?

The importance of a strict nuclear inspection regime in Pakistan is underlined by the growing danger that Saudi Arabia will seek to obtain Pakistani nuclear technology and weaponry in the increasingly likely event that Iran should become a nuclear power.

In a recent interview, Charles Freeman, former U.S. ambassador to Riyadh, said that King Fahd had told high-level U.S. officials on several occasions that Saudi Arabia would need a nuclear deterrent if Iran does develop nuclear weapons. At that time, Freeman said, the king envisaged a U.S. nuclear umbrella.

Given the tensions between Washington and Riyadh over the Iraq invasion, Riyadh is now likely to loosen, not tighten, its military ties to the United States, and Islamabad would be a more plausible nuclear partner for Riyadh than Washington.

On August 2, 2002, the Saudi defense minister was taken on a tour of Pakistani nuclear facilities, and lesser Saudi defense officials have visited Pakistani nuclear sites on two other occasions. Perhaps the most compelling argument for a U.S. nuclear inspection regime in Pakistan is that its nuclear facilities are riddled with Al-Qaida sympathizers who might smuggle fissile material out to terrorists.

In return for intrusive nuclear inspections, the United States should be prepared to offer Pakistan compelling new economic incentives, including access to the U.S. textile market, which it has been seeking in vain since it signed on as a U.S. ally after 9/11.

Stepped-up textile exports to the huge U.S. market would be an economic bonanza for Pakistan. Offering this would give the United States the leverage it needs for a showdown with Islamabad on nuclear inspections. President Bush has been reluctant to confront U.S. protectionist interests opposed to letting in Pakistani imports, but he should be willing to spend some of his political capital on this issue.

Stopping nuclear non-proliferation is a paramount U.S. interest, no less important than combating Al-Qaida.

Selig S. Harrison, director of the Asia Program at the Center for International Policy in Washington, is the author of ``Korean Endgame.''