PAKISTAN INVITES ridicule with its protestations that it is not building nuclear weapons. The Pakistani government asserts that it had nothing to do with the attempt to smuggle 25 tons of highly specialized steel from the United States to Pakistan. The steel is suitable for equipment to enrich uranium, and the case came to light two weeks ago with the arrest of a suspect in Philadelphia. In Islamabad, Pakistani officials called it a rogue operation for which the government bears no responsibility.

Really? It was hardly an isolated case. A few days after the Philadelphia arrest, a grand jury in California indicted three people for illegally exporting electronics equipment of types that a nuclear program would require. Again, the destination was Pakistan.

As for the steel, a smaller amount of the same special alloy was recently exported illegally from West Germany to Pakistan, Leonard Spector points out in his book, "Going Nuclear." According to German press reports the shipper sent the invoice and the bill to the Pakistani Embassy in Bonn. The steel was fabricated to precisely the specifications required by the designs stolen in 1975 by a Pakistani scientist, A. Q. Khan, from the European enrichment consortium where he was then employed. Dr. Khan is now head of the Pakistan government's enrichment program.

Pakistan promised the United States three years ago that it would not enrich uranium beyond the low level required to run civilian power reactors. There is now much evidence to suggest that Pakistan is pushing the enrichment level up to weapons grade and is expanding its weapons capacity.

American law forbids foreign aid to countries that build nuclear weapons. But currently the United States is providing very substantial aid to Pakistan, especially to keep open the supply routes to the guerrillas fighting the Soviets in Afghanistan. That confronts the United States with a difficult choice.

"Clearly, the outcome depends to a very large degree on Pakistan's response," Assistant Secretary of State Richard W. Murphy told a House hearing last week. Denials and denunciations issued from Islamabad won't suffice. If Pakistan wants to demonstrate its good faith, the way to do it is obvious. Most of the world's governments, including the United States and the Soviet Union, have opened their peaceful nuclear facilities to international inspection. If Pakistan's enrichment activities are innocent and limited, as it claims, it should have no objection to inspection. But if it continues to refuse anything more than the usual verbal assurances, the United States will have little choice but to enforce its law