Pakistani Prime Minister Mohammed Khan Junejo said yesterday his country has rejected a proposal that the Soviet Union withdraw its troops from Afghanistan over a four-year period and has insisted on a pullout timetable of four months or less instead.

Junejo's account of the closely held Geneva negotiations on the future of Afghanistan was the most authoritative and specific yet made public. It suggested a wide gap on the central issue of Soviet withdrawal that is likely to be difficult to overcome.

"We don't see any sincerity" in the offer for a four-year Soviet troop pullout that was presented by the Afghan government in the Geneva talks two months ago, Junejo told reporters and editors of The Washington Post. Nevertheless, he said Pakistani Foreign Minister Sahabzada Yaqub Khan will continue the U.N.-sponsored indirect talks with the Afghan government in Geneva July 30 in hopes of making progress.

When the four-year Soviet pullout proposal was advanced by Afghanistan, the prime minister said in recounting what happened in mid-May, Khan "sharply reacted and said, 'If this is the way you'd like to handle the talks it will be difficult for me to sit here even to negotiate for a minute because I don't see any sincerity in this offer.' "

Junejo said that after this exchange his government continued the Geneva discussions essentially out of courtesy to the United Nations, whose successive secretaries general have sponsored the negotiations.

The mid-May round of talks, Junejo noted, marked the first time the Soviets and the Soviet-backed Afghan regime had ever been willing to specify any timetable at all for withdrawal of the 120,000 Soviet troops. The prime minister attributed the Soviet willingness to permit discussion of the issue to international condemnation of Moscow's invasion and occupation, and to domestic unhappiness about the high casualties being suffered by Soviet troops.

Among the Soviet pressures exerted on Pakistan in connection with the negotiations and the Afghan situation in general, according to Junejo, are increasing instances of border incursions, bombings and sabotage against Pakistani territory.

In addition, Junejo said at a Pentagon appearance later in the day, the Soviet Union "sent us a crude warning and threat" last month, demanding that Pakistan change its Afghan policy. "We will not be intimidated" but at the same time Pakistan cannot ignore the Soviet warning, Junejo said.

The Soviet Union has charged that Pakistan, with U.S. direction and support, is aiding guerrilla forces fighting Soviet and Afghan troops in Afghanistan.

Pakistani sources said the "warning and threat" that Junejo mentioned was part of the same diplomatic protest from Moscow in the third week of June that also attacked Pakistan's nuclear program. The Soviets reportedly charged in that message that Pakistan has "reached the capacity" of making a bomb, which they regard as a threat to "the southern part of the U.S.S.R."

The Soviets warned that "they could not be indifferent" to this nuclear danger, according to sources. Pakistani officials concluded that the nuclear issue was being used as pressure against Pakistan on the Afghan question, the sources said.

Junejo, in the Post interview, said, "It's quite interesting that a superpower like the Soviet Union feels its concern about the nuclear issue of Pakistan." He said his government told Moscow -- as he said he has told President Reagan and other U.S. officials here this week -- that Pakistan's nuclear program is "purely for peaceful purposes" and that "we have no intention of going for nuclear weapons."

The prime minister confirmed that Pakistan pledged in response to a 1984 letter from Reagan not to enrich uranium in its nuclear facilities to a level higher than 5 percent. Such uranium is useful in nuclear power and other civil purposes but is far short of the 90 percent enrichment needed for weapons.

Junejo said Pakistan's enrichment facility hasn't reached "even 5 percent" in its operations, denying reports published in London that the plant at Kahuta has enriched uranium to 30 percent or more.

The United States and Pakistan, in a White House ceremony yesterday, signed an agreement permitting Pakistan to buy advanced U.S. technology. U.S. officials said it included tough guarantees banning its use in Pakistan's nuclear program.