ISLAMABAD, PAKISTAN, JULY 21 -- Senior government officials acknowledged today that Pakistani citizens may have been involved in attempts in the United States to smuggle illegal materials used in making nuclear weapons, but denied that such efforts had the support or approval of President Mohammed Zia ul-Haq's government.

A highly placed government source said tonight that an arrest warrant has been issued for a Pakistani national in connection with charges brought two weeks ago in federal court in Philadelphia, implicitly acknowledging Pakistani involvement in the case for the first time.

The official portrayed the action, however, as a rogue operation that ran counter to Pakistani government orders forbidding the violation of U.S. laws against the export of goods used in the manufacture of nuclear weapons.

The official argued vehemently that Pakistan had done no wrong and that the furor raised in Congress by the charges is serving only to undermine Pakistani confidence at a turbulent time in the politics of the region, which stretches from the Persian Gulf to South Asia.

The Pennsylvania case, and a subsequent charge in California, have led to demands in Congress for a cutoff of the multibillion dollar U.S. aid program to Pakistan. Officials here describe the situation as probably the most critical moment in U.S.-Pakistani relations since the two countries began cooperating closely after the 1979 Soviet invasion of neighboring Afghanistan.

A cutoff in U.S. aid would be likely to trigger a Pakistani reassessment of its support for the Afghan resistance opposing the Soviet-backed government in Kabul and its own relationship with the United States.

So far, the desire in the United States to maintain pressure by the Afghan guerrillas on the Soviet Union and Afghanistan has overridden demands by congressmen to halt aid to Pakistan. However, the latest case appears to have shifted the balance, at least temporarily.

"We have launched a very high-level probe," the official here said. "It is possible that a Pakistani already has been identified as participating in some kind of a conspiracy.

"Pakistan is quite clear that no agency of the government should be engaged in an activity that violates U.S. laws. We don't want to import anything that might be suspect. We are looking into why this person ordered this material. It is not quite clear why this person authorized going to the U.S. {One can} get it where it is legally available. Why go to the U.S.?"

According to the charges and supporting material filed in federal court in Philadelphia, Arshad Z. Pervez, a Pakistani living in Canada, sought to buy 50,000 pounds of maraging 350 steel from Carpenter Steel Corp. in Reading, Pa., and offered to bribe an undercover agent to get the necessary export licence.

The steel is a special alloy of extremely high strength that can be used in nuclear enrichment facilities.

According to the documents, Pervez was working on behalf of a Pakistani named Inam ul-Haq, a retired Army brigadier living in Lahore, Pakistan, who owns a company called Multinational Corp.

{Administration officials and diplomatic sources in Washington said they understood the warrant was issued for Inam ul-Haq, Washington Post staff writer Don Oberdorfer reported.}

A fake export license was issued by the Commerce Department and Pervez subsequently was arrested after the bribe money allegedly was paid.

The report of the arrest sent shock waves through the Pakistani government, which thought it had successfully weathered an intensive debate in Congress over a new $4 billion aid program and this country's nuclear program. U.S. laws prohibit aid to a country seeking to acquire U.S. material illegally to develop a nuclear weapon.

President Zia ul-Haq and Prime Minister Mohammed Khan Junejo reportedly assured U.S. Ambassador Arnold Raphel after the case was filed in court that strict orders had been given months ago to prohibit any purchases in the United States that could cause political trouble for the aid package.

They reportedly expressed the belief that the charges must be part of an attempt to undermine the aid package, probably organized by India.

"Strict orders went out. They knew how much trouble it would cause in Congress at this stage," said one person familiar with the high-level discussions.

The apparent decision to prosecute Pakistanis in the case indicates either that the assurances to Raphel were false or that there was a severe breakdown in the Pakistani chain of command. The latter interpretation is given some credence by knowledgeable observers here, although it is unclear so far just where the breakdown occurred and whether Pakistani actions to prosecute would satisfy a highly skeptical Congress.

Since the maraging steel apparently was intended for the top secet nuclear facility at Kahuta, where Pakistan has developed the capacity to enrich uranium by a gas centrifuge process, there is some belief among experts that the controversial scientist A.Q. Khan could be involved in the case. He is believed to be responsible for the Kahuta facility.

Khan was involved in a controversial interview with an Indian journalist a few months ago in which he said Pakistan had the capacity to build a bomb, an assertion that nearly threw legislation in Congress off the tracks.

"He basically is not taking orders. He is a megalomaniac. It is not the first time," said one expert, who nevertheless expressed doubts that the Pakistani government would move against the man viewed as the father of Pakistan's nuclear program.

"He is a national folk hero, sort of like Ollie North in the U.S.," said the expert.

The Pakistani official said today only that "any individual in the service of the government of Pakistan who is found to have violated the clearly spelled out policy prohibiting procurement in the U.S. will be prosecuted." The charge likely would be conspiracy to harm Pakistan's national interest, since there are no laws here against importing maraging steel.