The Bush administration has evidence that suggests Pakistan assisted North Korea's covert nuclear weapons program as recently as three months ago, much later than previously disclosed, according to sources in the administration and on Capitol Hill.

While the administration has taken a hard line against North Korea, demanding that it verify it has dismantled its efforts to enrich uranium before U.S. officials engage in further discussions with the communist state, it has taken a much softer tack against Pakistan. Publicly, officials have suggested that if Pakistan, a key ally in the war against terrorism, had provided help to North Korea in the past, it changed its behavior after the Sept. 11, 2001, attacks in New York and Washington.

But in reality, U.S. officials say, the administration believes Pakistan continued to trade nuclear technical knowledge, designs and possibly material in exchange for missile parts up until this summer, when the administration concluded North Korea was secretly trying to construct a facility to enrich uranium for a bomb. Administration officials would not discuss the extent of the evidence, but they said it involves highly suspicious shipping trade.

"Let's put it this way: There were still shenanigans going on three months ago," an administration official said. Intelligence officials who have briefed members of Congress have also disclosed the administration's concerns that Pakistan's illicit nuclear trade continued well into this year.

Pakistan's involvement in North Korea's program has put the administration in an extremely delicate position. Under U.S. law, if the president determines that a country has delivered nuclear enrichment equipment, material or technology without international safeguards, the United States must suspend economic and military aid. Such sanctions were imposed against Pakistan in 1979, but last year President Bush waived them and other nuclear-related sanctions after the Pakistani government agreed to help in the fight against al Qaeda and Afghanistan's Taliban militia after the Sept. 11 attacks.

Rather than press Pakistan for a full accounting, U.S. officials said they have noted the latest evidence -- which Pakistani officials have argued is innocent -- and believe they have put Pakistan on notice that future violations will not be tolerated. Intelligence officials plan to closely scrutinize transactions between Pakistan and North Korea.

Pakistani President Pervez Musharraf has personally guaranteed that questionable transactions with North Korea will cease, and U.S. officials believe he would like to halt the nuclear leakage. But they also question whether he has full control of all entities that could be doing business with North Korea. "In the end, we may find he is only partially truthful," the official said.

Several experts said it will be difficult to understand the scope of the North Korean program -- which by some estimates would not be operational for several years -- unless the administration demands that Pakistan disclose exactly what it might have provided to North Korea.

"We have asked North Korea to verifiably dismantle its nuclear enrichment program," said Robert J. Einhorn, former assistant secretary of state for nonproliferation in the Clinton and Bush administrations and now a senior adviser at the Center for Strategic and International Studies. "How will we know if North Korea has done that unless we know precisely what Pakistan has transferred to North Korea?"

Pakistani officials publicly insist that they have not helped the North Korean program in any way.

"No material, no technology ever has been exported to North Korea," said Ashraf Jehangir Qazi, the Pakistani ambassador to the United States. "I can assure you there is no way we would underestimate the seriousness of such an international breach."

Qazi said that while Pakistan has engaged in trade with North Korea, "nobody can tell us if there is evidence, no one is challenging our word. There is no smoking gun."

Last month, U.S. officials confronted North Korea with their conclusion that it had a covert nuclear program. Then, North Korea unexpectedly admitted it.

Pakistan produces highly enriched uranium for nuclear weapons, and U.S. officials have long suspected that Pakistani nuclear scientists had disturbing ties to the North Koreans.

In the face of Pakistan's vehement denials, U.S. officials have been publicly anxious not to suggest that Musharraf, who seized power in 1999 in a bloodless coup, is anything but a close friend and ally.

Indeed, asked last month about reports that Pakistan provided assistance to North Korea's program, White House spokesman Ari Fleischer did not confirm the reports but noted: "Many things that people may have done years before September 11th or some time before September 11th, have changed. September 11th changed the world and it changed many nations' behaviors along with it."

Secretary of State Colin L. Powell has been careful publicly not to suggest when Pakistan may have helped North Korea. Instead, he said that as recently as last month, he spoke to Musharraf "about the need not to assist North Korea in any way and have any kind of relationship with North Korea now that would give them the wherewithal to develop those kinds of weapons or the means to deliver them."

Powell said he purposely did not dwell on past behavior because "the past is the past and there isn't a whole lot I can do about it. I'm more concerned about what is going on now. We have a new relationship with Pakistan."

Leonard Weiss, a former Senate staffer who specialized in nonproliferation issues, said there is "no question" that, under a 1976 law known as the Symington amendment, Pakistan would qualify for sanctions if it aided North Korea's program. But he said that if officials decide not to probe too deeply, "they avoid the political problem of having to give them a waiver."