A senior State Department official said yesterday that President Reagan cannot assure Congress with complete reliability that Pakistan is not building a nuclear bomb or that it is not producing weapons-grade uranium.

The official nonetheless urged legislators to exercise "utmost caution and discretion" in its deliberation over termination of U.S. assistance to Pakistan because of U.S. intelligence reports that Pakistan is enriching uranium well above the 5 percent level needed for peaceful purposes.

Testifying before three House subcommittees, Ambassador-at-Large Richard T. Kennedy warned that the United States could "lose the ball game" by provoking Pakistan into building and testing a bomb if U.S. aid is cut off. The likely result, he said, could be a nuclear arms race between Pakistan and India that Congress has been seeking to head off.

Kennedy, who is Secretary of State George P. Shultz's chief adviser on nuclear nonproliferation policy, confirmed that the administration believes there has been "some occasion" in which Pakistan has gone beyond the 5 percent enrichment level.

But he argued this should not be taken as a final indicator it intends to build a bomb. "The ball game is lost when in fact they cross the threshold and put together a weapon and in fact explode one," he said.

The issue of what level of uranium enrichment Pakistan has reached at its Kahuta plant has taken crucial importance because of legislation being introduced in the House by Rep. Stephen J. Solarz (D-N.Y.) and in the Senate by Sen. John Glenn (D-Ohio).

The new measure would require a U.S. aid cutoff to Pakistan after six months unless the president determines "on the basis of the best available information" that Pakistan has not exceeded the 5 percent mark.

The administration has proposed a new six-year, $4 billion military and economic aid package for Pakistan.

But ever since a Pakistani native was arrested in Philadelphia in July and charged with attempting the illegal export of nuclear-related equipment to Pakistan, Congress and the White House have struggled to reach agreement on the issue of Pakistan's nuclear intentions before any new assistance can flow. Mujaheddin resistance groups fighting the Soviet army in Afghanistan receive massive U.S. arms aid through Pakistan.

Under questioning from Solarz and Rep. Howard E. Wolpe (D-Mich.), Kennedy acknowledged that the administration has "some reservations" that Pakistan's recent assurances it is not enriching above the 5 percent level could meet "the standards of reliability" Congress may now insist upon.

Asked what he understood by this standard, Kennedy replied, "The highest level of assurance, without any doubt at all."

Congressional criteria demand that the president receive "reliable assurances" that a nation is not developing nuclear weapons in order for him to waive a congressional ban on aid to countries suspected of obtaining nuclear enrichment materials or equipment.

Such assurances imply international on-site inspection, which Pakistan refuses to accept unless India also accepts it.

"We believe that the standard which would be required for a presidential determination would perhaps be of an order that we simply could not {fulfill}," Kennedy said.