President Reagan certified to Congress last week that Pakistan currently "does not possess a nuclear explosive device," although according to intelligence reports considered reliable inside the administration, the country has recently made dramatic progress toward production of a nuclear weapon.

Presidential certification is required by Congress as a condition of continued U.S. aid to Pakistan, which receives approximately $ 600 million a year in military and economic assistance. Pakistan has cooperated with clandestine U.S. aid to Afghan guerrillas fighting the Soviet occupation of their country, and has provided facilities for U.S. intelligence-gathering near the Soviet Union.

According to a classified Defense Intelligence Agency report, Pakistan detonated a high explosive device between Sept. 18 and Sept. 21 as part of its continuing efforts to build an implosion-type nuclear weapon, sources said.

It was Pakistan's second such test this year, according to the sources, who said the Pakistanis have been conducting the tests for years in trying to perfect a nuclear weapons triggering package.

Intelligence reports also show that Pakistan has enriched uranium to 93.5 percent at its atomic plant at Kahuta, according to authoritative sources. A 90-percent level is normally needed to make a bomb. President Reagan in late 1984 told Pakistani President Mohammed Zia ul-Haq in a top-secret letter that 5 percent would be the highest enrichment level acceptable to the United States.

In July, the White House warned Pakistani Prime Minister Mohammed Khan Junejo during his visit here that acquiring a nuclear weapon would result in the end of U.S. economic and military assistance.

Pakistan has repeatedly denied that it is developing nuclear weapons. But a Special National Intelligence Estimate (SNIE) completed earlier this year by U.S. intelligence agencies cited numerous activities totally inconsistent with those assurances, according to sources. The SNIE concluded that Pakistan would have a small nuclear weapon at a future, unspecified date.

Charles E. Redman, the State Department spokesman, said yesterday that Reagan signed the Pakistani certification Oct. 27. He added, however, that the Reagan administration still has "serious concerns" about the future and said the current certification should "not be interpreted as implying any U.S. approval of the Pakistani nuclear program."

He declined to comment on any intelligence reports.

There is disagreement among intelligence and nuclear nonproliferation experts about the exact status of the Pakistani program. One senior Reagan administration official confirmed that the program is advancing aggressively, but said that a new, multibillion-dollar U.S. assistance proposal would provide leverage to deter actual construction of a bomb.

Another official said Pakistan could assemble a bomb within two weeks. Another well-informed source said it could be done in a shorter time and, in practical terms, Pakistan is only "two screwdriver turns" from having a fully assembled bomb.

Despite this evidence, sources said, keeping Pakistan from obtaining a bomb is a low priority on the list of administration foreign policy goals. Said one senior official directly involved in monitoring the program, "This administration wouldn't come down on Pakistan if we found a bomb in Zia's basement."

The reason, the sources said, is Pakistan's willingness to help the administration by acting as the pipeline for the hundreds of millions of dollars in CIA covert assistance that is provided the Afghanistan rebels -- a top priority for Reagan and his administration. At the time of Junejo's visit last summer to Washington, Reagan said that Pakistan was a "front line" against "the brutal Soviet occupation of Afghanistan."

Pakistan also cooperates with U.S. intelligence agencies in high-priority electronic intelligence gathering near the Soviet Union and in Southeast Asia, the sources said.

On June 21 the Soviets issued a strongly worded, unusual warning to Zia charging that Pakistan had achieved the capability to build nuclear weapons, which Moscow said it would not tolerate, according to sources.

Within two days, the Reagan administration replied with its own protest, in effect telling Moscow to keep "hands off" Pakistan. This in effect made the administration a protector of the Pakistani program, and two sources said that the Pakistanis may have interpreted the adminstration's remarks as approval.

A senior administration official disputed this interpretation, however, saying the White House made clear last summer to Junejo that a single bomb would result in termination of all U.S. aid.

The intelligence report that uranium has been enriched at levels in excess of 90 percent has alarmed nuclear weapons experts most. Leonard S. Spector, a nuclear proliferation expert at the Carnegie Endowment for International Peace, said yesterday that, if true, "it would be the last important step in the Pakistani program. It was the one outstanding gap in their program and could be a terrible setback to worldwide efforts to curtail the spread of nuclear weapons."