A month after the Soviet Union issued a strongly worded and unusual warning to Pakistan about that country's nuclear program, U.S. and Pakistani officials are still puzzled about the timing, tactics and main purpose of Moscow's move.

The visit here last week of Pakistani Prime Minister Mohammed Khan Junejo included considerable discussion in private and some in public of Moscow's nuclear warning, which was delivered June 21 by the Soviet ambassador in Islamabad to President Mohammed Zia ul-Haq.

No U.S.-Pakistani consensus has been reached about what lay behind Moscow's move, according to U.S. officials who participated in last week's talks. There is widespread agreement that the injection of this element adds to the complexity of the volatile geopolitics of southwest Asia.

During periodic U.S.-Soviet meetings on nuclear proliferation, the two superpowers, which share an aversion to the spread of nuclear weapons to smaller nations, have often discussed Pakistan's decade-old atomic program. Until now, however, the Soviets have not emphasized the case of Pakistan nor displayed unusual concern about it, according to U.S. sources.

Washington officials say they hope that they will learn more about Soviet views on July 28, at the next meeting of U.S. and Soviet antiproliferation experts.

What is known is that, in their June 21 protest, the Soviets charged that Pakistan had achieved the capability of making nuclear weapons, declared that this poses a threat to "the southern part of the U.S.S.R." and concluded that Moscow "cannot be indifferent" to this development.

It is possible, officials concede, that Soviet intelligence has picked up reports similar to those reaching Washington in recent months indicating major strides in the Pakistani atomic program. The reports suggested that Pakistan, despite official denials, may be at or over the threshold of nuclear weapons capability.

Intense Soviet concern about nuclear weapons on its borders led Moscow to break off nuclear cooperation with the People's Republic of China in the late 1950s, a major cause of the Sino-Soviet split. There were reports that Moscow considered a preemptive strike against Chinese nuclear facilities after China exploded its first atomic bomb in 1964.

U.S. and Pakistani officials have speculated that the June 21 Soviet protest was related to Soviet ties to India, which has increasingly expressed concern about Pakistan's nuclear capabilities. Indian Foreign Minister P. Shiv Shanker met with Soviet leader Mikhail Gorbachev in Moscow June 14, one week before the Soviet message to Pakistan, a fact that has encouraged the "India theory" of the Soviet action.

The Soviet warning also came just weeks before the Pakistani prime minister was to depart for last week's visit to Washington and not long before the scheduled resumption of indirect talks between Pakistan and Afghanistan July 30 on the Afghan situation. The timing of the latter event -- and the fact that the Soviet note challenged Pakistan's aid to Afghan rebels as well as its nuclear policies -- suggested to some Pakistani officials that Moscow was seeking to use the nuclear issue as new pressure on the Afghan question.

Pakistan's Zia sent an immediate message about the Soviet warning to Washington through Foreign Minister Sahabzada Yaqub Khan, who was here at the time. Yaqub Khan took it personally to Vice President Bush. Within 48 hours, according to administration sources, the United States had issued a counter-warning, in effect telling Moscow to keep "hands off" Pakistan.

Thus, Washington has been positioned as the defender of Pakistan's nuclear program against challenge from Moscow, even though the United States has been concerned for years about Pakistan's drive to acquire atomic weapons.

The developments also have emphasized the increasing dangers to the great powers as well as regional powers of nuclear developments in southwest Asia. While Pakistan has been moving toward atomic weapons capability through its giant uranium enrichment plant at Kahuta, India has begun producing weapons-grade plutonium from a reactor near Madras -- enough for at least one bomb, according to Leonard S. Spector, nuclear proliferation expert at the Carnegie Endowment for International Peace. This Indian reactor, like the Pakistani enrichment plant, is not subject to international safeguards.

Spector said after the Junejo visit here that the technical developments in Pakistan and India have "outpaced diplomatic efforts" to deal with the growing nuclear threat in that part of the world.