The Reagan administration, facing crucial votes on aid to Pakistan, has withheld information from Congress that Pakistan has completed one of the last two steps for producing nuclear bombs, Senate Assistant Minority Leader Alan Cranston (D-Calif.) charged yesterday.

The Pakistanis now can produce weapons-grade plutonium without detection by international monitors, Cranston said, leading Sigvard Eklund, head of the International Atomic Energy Agency, to complain that his agency no longer has confidence in its accountability safeguards. India has the same capacity.

To produce an atomic bomb, Pakistan needs only to develop a warhead, Senate aides said.

"I am deeply concerned by these developments, which represent another in a series of relentless steps toward a nuclear arms race on the South Asian subcontinent," Cranston told the Senate Foreign Relations Committee.

His statements came only days before the Senate is to vote on repealing a ban on aid to Pakistan. Lifting the ban would clear the way for a $3.2 billion aid and sales agreement that includes sale of 40 F16 fighters.

The administration wants repeal of the ban, which was passed in 1979 because of fears of nuclear proliferation. The Senate is sympathic to lifting the ban, but Sen. John Glenn (D-Ohio) plans to offer aid restriction amendments.

Cranston made his allegations at a confirmation hearing for Harry Barnes and Ronald Spiers, who have been nominated as ambassadors to India and Pakistan, respectively. Both are career foreign service officers.

"India and Pakistan, two nations that don't have the greatest relations with each other, now have the potential sources for undetected plutonium diversion to facilitate nuclear weapons operations," Cranston said.

Spiers, director of the State Department's intelligence and research bureau, confirmed that Pakistan is making nuclear fuel at a Canadian-supplied reactor near Karachi. He said he does not know why the administration did not inform Congress of the activity.

Spiers said the international atomic agency "has not determined that any violations of the safeguards has occurred in Pakistan, that any nuclear naterial has been diverted or that there has been a noncompliance with safeguard arrangements."

Despite assurances that it has no intention of developing nuclear weapon, Pakistan has rapidly marched toward doing so, Glenn and other senators said yesterday.

The development Cranston cited is signficant because the amount of uranium going to the Canadian-built reactor is monitored, since Canada abides by nuclear proliferation safeguards.

Pakistan does not abide by these safeguards, and any plutonium it produces on its own would not come under international monitoring.