Defense Secretary Harold Brown issued a warning today to the NATO nations that the Soviet nuclear advantage in Europe will become "unmanageably large" unless the West European alliance upgrades its own nuclear arsenal.

Brown and other NATO leaders here appeared to be trying to influence the political climate among NATO allies toward acceptance of newer and additional nuclear weaponry now being planned for the theater.

They hope to avoid a repetition of the controversy last year that blocked deployment of the neutron warhead in Europe.

Brown said the NATO leadership hopes to reach a general decision by year-end on what additional nuclear weapons to add to the European inventory to counter such Soviet systems as the SS20, a landbased mobile missile with 3,000 mile range, and the Backfire bomber, which would not be limited by the pending strategic arms limitation (SALT) draft treaty.

As an example of the sensitivity of the issue, West German Defense Minister Hans Apel told the NATO Nuclear Planning Group's 25th annual meeting that any new deployment of nuclear weapons in his country must be an alliance-wide undertaking, not a "bilateral" deal between Washington and Bonn.

Widespread political concern surfaced in Germany during the neutron warhead furor that because of the positioning of nuclear weaponry on its soil it might be singled out as a special target in any future hostilities. Apel's comments were delievered in a separate session with his own country's press.

If Germany does allow the United States to deploy the new 1,500-mile-range Pershing II nuclear missile on its soil-a weapon that could hit the Soviet Union-the Soviets almost certainly would train more of their own nuclear fire on Germany.

Gen. Alexander M. Haig, NATO commander in Europe, wants to spread the modernization among the NATO partners, thus muting the political effect. For example, if Germany accepted a Pershing II, Haig sees advantages to deploying cruise missiles targeted on the Soviet Union in other European countries, such as Britain and France.

Haig said in an interview here with The Washington Post today that he favors keeping all options open: the Pershing II; cruise missiles for launchers on land, airplanes and in submarines, and development of a new "medium or intermediate" range missile for NATO as a specific counter to the Soviet SS220.

Haig said that cruise missiles in submarines might turn out to be more "politically comfortable" to NATO partners than land-based cruise missiles and thus the sea-basing option should be kept open. Right now, sea-based cruise missiles are far down on the Pentagon's list of priorities.

Asked about the political problems stemming from the Germans inviting more fire on themselves by deploying the Pershing II on their soil, Haig replied:

"They're targeted now" by Soviet SS20 missiles. "They would be better off if they were able to respond."

The United States believes the Soviets have deployed more than 100 SS20s, each carrying three nuclear warheads that can hit different targets.

The public remarks by Brown and the final communique of the 25th annual meeting of the Nuclear Planning Group are part of a campaign to sell NATO nation on the need to deploy additional nuclear weapons in Europe, even though some critics contend that there are too many there already and that the firing of a little "nuke" would surely result in an escalation to full-scale nuclear war.

Brown, at the press conference, predicted that the Soviets would wage a "propaganda campaign" against NATO efforts to modernize its feeder nuclear weapons. "The Soviet Union is against all improvements in western capabilities," Brown said.

In discussing the pending SALT II agreement, Brown repeated his earlier contention that the United States "can verify adequately" whether the Soviet Union in its missile developments was obeying the rules.

"I deny our misleading anyone" in making that assertion, Brown said. He was responding to such a charge made recently by Sen. Jake Garn (R-Utah).

Garn, continued Brown, has made it clear that he would be aginst the SALT II agreement even "if verification were perfect."

Besides Brown, and Apel, other representatives attending the conference here were: Joseph Luns, secretary-general of NATO, and defense ministers or their representatives from Italy, the Netherlands, Norway, Britain, Canada and Turkey. CAPTION: Picture, West Germany's Apel gestures to Brown and others leave NATO defense ministers meeting. AP