The dirty yellow cloud of poisonous gas has supplanted the atom's mushroom cloud as a symbol of the most pressing proliferation danger facing the world, in the view of government officials from the United States and several other countries.

While no nation has joined the A-bomb club since India conducted a nuclear test in 1974, the deadly chemicals known as "the poor man's atomic bomb" have been repeatedly used in warfare in the 1980s, and in ways that experts fear may promote their further use.

In an effort to stem the tide, officials and chemical specialists from the United States and chemically advanced Western European and Asian countries held an unpublicized meeting for several days last week in Brussels, under the leadership of Australia, to discuss ways to prevent the production and use of chemical weapons from spreading to additional countries. This was the second meeting since June of this group, whose existence is so sensitive with some governments that it has not been given a name.

Secretary of State George P. Shultz said earlier this year that the United States thinks that at least 13 nations have chemical weapons, compared with five in 1963, and that additional nations are trying to get them.

"The sad fact," Shultz said, "is that a half century of widely accepted international restraint on the use or development of chemical weapons is in danger of breaking down."

Other U.S. officials have said that at least 15 countries belong to the "chemical weapons club."

"Proliferation is an enormous problem," said a senior State Department official who has been deeply involved in low-key U.S. efforts to limit them. "I'm afraid that the number [of chemical weapons nations] could double in the next decade."

Since Iraq used mustard gas and nerve gas against Iranian troops in early 1984 and again this year, concern has mounted, generating U.S. interagency studies, chemical-export controls and unpublicized international meetings with American allies to consider joint actions.

The most acute worry is that a future Iranian offensive will trigger another Iraqi poison gas attack and that, in retaliation, major Iranian gas attacks will be launched on the battlefield or against civilian targets. Such an exchange would be the first time since World War I that both sides have used chemical weapons in a war.

Officials are also concerned that if Iran uses chemical weapons it might also supply poison gas to terrorist groups.

Recent U.S. and international discussions have covered such items as restricting shipments of "precursor chemicals" that could be used in chemical weapons and creating "trigger lists" of chemicals whose acquisition should set off alarms in world capitals. The antiproliferation program in the chemical-weapons field is in its infancy, however, compared with the extensive international drive to halt the spread of nuclear weapons.

"Unless we in the West and others get our act together soon to stop the spread of chemical weapons, we will pass up a good opportunity," said Kenneth L. Adelman, director of the Arms Control and Disarmament Agency. "We can possibly nip this looming threat early, before chemical weapons become as commonplace as hand grenades in Third World armies."

Prof. Joseph Nye of Harvard University, who served from 1977 to 1979 as the key U.S. negotiator in creating a "suppliers' group" of advanced nations working together against nuclear weapons proliferation, said the drive against chemical weapons is "not even as far along" and that it faces considerably more difficult problems.

Nye said it is more difficult to obtain a broad political consensus against chemical weapons, which lack the "species threatening" dimension of atomic weapons. For example, the Soviet Union, which has cooperated in the effort to control nuclear-weapons proliferation, is considered a big part of the problem in the proliferation of chemical weapons.

Moreover, chemical weapons are much easier to manufacture -- and thus more difficult to control -- than nuclear weapons.

Particularly worrisome, Nye said, are growing programs here and in the Soviet Union to investigate bioengineering, especially the creation of potent new biological substances, as a weapon of war.

The fields of chemical and biological warfare are governed by separate international agreements, but are closely related. The distinction is that biological weapons are living organisms, while chemical weapons are not. Falling in a middle ground are toxins such as "yellow rain," described by the United States as a chemical byproduct of biological processes.

Mounting concern about the spread of chemical weapons in Third World nations comes as a 40-nation conference in Geneva continues to work on a new worldwide chemical weapons ban, without notable success, and as the United States appears about to resume production of nerve gas for its chemical-weapons stockpiles.

Production was halted by President Richard M. Nixon in 1969, but the Reagan administration has waged a three-year battle to restart it.

After a major fight, a House-Senate conference committee authorized resumption of poison-gas production in July, and an appropriation to supply the money is pending on Capitol Hill.

Resumed U.S. production of chemical weapons "could well promote proliferation" by other nations, said a 1984 study by the Congressional Research Service, "but if it does, it will be one factor among many doing so."

The extensive use of gas warfare in World War I generated worldwide revulsion that led to the Geneva Protocol of 1925 outlawing the use of chemical and bacteriological weapons. The protocol, signed by 106 nations, does not outlaw the development or possession of such weapons, only their use. Nonetheless, it eased fears about the weapons for most of the years since.

The Japanese reportedly used gas in Manchuria. And the Italians were said to use it in Ethiopia in World War II. Egypt is thought to have used mustard gas against Yemen in 1963 and 1967. These attacks against unprotected troops or populations were limited in their international impact, however.

Chemical warfare drifted back into the headlines in September 1981, when the Reagan administration charged the Soviet Union with using poisonous "mycotoxins" -- popularly known as "yellow rain" -- in southeast Asia. Later the administration charged the Soviets with supplying traditional chemical weapons and "yellow rain" toxins that were used in Afghanistan as well as Laos and Cambodia.

The "yellow rain" charges were and are disputed by a number of private scientists, but the administration continues to reaffirm them. Last month, the State Department said that after "extensive review and analysis by independent authorities in the field as well as govermment experts . . . our conclusions stand. Chemical and toxin weapons have been used in southeast Asia and Afghanistan."

U.S. officials said, however, that reports suggest that use of chemical weapons and "yellow rain" in those areas has greatly diminished or stopped in the past year or two.

Whatever doubt still exists about "yellow rain," there is little doubt about Iraq's use of chemical weapons against Iranian ground troops in February to April 1984, and again this spring. In both cases, U.N. reports and other independent studies backed up the charges, and Iranian soldiers suffering from poison-gas attacks were treated at hospitals in Western Europe.

"Iraq has gotten away with the use of chemical weapons with minor costs," said Brad Roberts, an expert at the Georgetown Center for Strategic and International Studies. He said this is likely to spur the acquisition and use of poison gas by other countries because "Third World defense planners can see that Iraq turned back a major offensive by Iran with chemical weapons" but hasn't seemed to suffer for it.

Iran has threatened to retaliate in kind. And on April 24, the State Department said Iran "has been seeking to develop a chemical-weapons capability and may now be in a position to use such a weapon." Late last month, a U.S. official familiar with the intelligence said, "Iran has the capability" to use chemical weapons. A few limited chemical attacks attributed to Iranian forces in the past, he said, apparently relied on Iraqi chemical shells captured on the battlefield.

Because chemical-warfare capabilities are shrouded in secrecy and nations rarely admit that they possess such weapons, clear-cut, well-confirmed facts are rare.

A February 1985 report in Chemical and Engineering News, which is said to reflect official information, listed four countries asconfirmed possessors of chemical weapons: the United States, Soviet Union, France and Iraq.

Eleven other countries were listed as those "alleged to possess" chemical weapons: Egypt, Syria, Libya, Israel, Ethiopia, Thailand, Burma, China, Taiwan, North Korea and Vietnam.

A September 1983 U.S. intelligence estimate from CIA and other sources, first made public by Jack Anderson and Dale Van Atta in August 1984, gave these details:

*Egypt received Soviet chemical-weapons training, indoctrination and materiel in the 1960s while it was the major Soviet client in the Middle East.

*Syria has "probably the most advanced chemical-warfare capability in the Arab world" with the possible exception of Egypt. As of 1983 no Syrian production facility had been identified and there was "no need" in view of chemical agents and delivery systems reportedly flowing from the Soviet Union and Czechoslovakia.

*Libya met with "little success" in obtaining chemical weapons plants from Eastern or Western Europe but may possess lethal chemical agents for "experimental purposes."

*Israel undertook a chemical-weapons program after capturing large amounts of Soviet-supplied equipment from its Arab foes in the 1967 and 1973 wars. Israel is thought to have "at least" nerve gas, mustard gas and riot control agents with "suitable delivery systems" and to have tested its weapons as early as 1976.

*Ethiopia acquired "chemical agents, munitions and decontamination equipment" from its Soviet ally. Reports of lethal Ethiopian attacks against Eritrean insurgents are "unconfirmed," although many U.S. officials consider them credible.

*Thailand, in response to a Vietnamese chemical-warfare threat, is "upgrading its capabilities" by improving its research and acquiring protective equipment from the West. U.S. officials said recently, however, they do not think that Thailand possesses offensive chemical weapons.

*Burma has been seeking since at least 1981 to produce mustard gas. The Central Intelligence Agency estimated that Burma should be "self-sufficient" in chemical weapons by the spring of 1984, most likely for use against internal insurgencies.

*China has a "small" offensive chemical-warfare capability. China is thought to have suffered gas attacks in a skirmish with Soviet forces in 1969 and Vietnamese forces in 1979.

*Taiwan has "an aggressive, high-priority program to develop both offensive and defensive capabilities." Taiwan has produced at least mustard gas, the report said.

*North Korea "reportedly stores and produces" crude chemical weaponry, but the reports are "unsubstantiated."

Vietnam's chemical-weapons capability, with "a range of agents" in addition to "yellow rain," is reported by U.S. officials to have been "transferred" to that country by its increasingly close ally, the Soviet Union.

Soviet forces are thought to have stockpiled chemical weapons in a number of Eastern European countries, and there is controversy in the U.S. intelligence community about whether these nations have their own production facilities. A West German official said his government thinks that East Germany, Czechoslovakia and Poland are producing chemical weapons.

A number of other nations, including South Korea, are reported to be interested in acquiring chemical weapons. A proposal that U.S. forces in South Korea be armed with chemical weapons -- in light of reports that North Korea has them -- is under study in the Pentagon.