Calling many of the 7,000 U.S. tactical nuclear weapons in Europe "largely obsolete," Sen. Sam Nunn (D-Ga.) said this week that NATO should approve a Carter administration program for new medium-range missiles or face the possibility that the entire nuclear force will be removed.

In the frankest public assessment yet of the aging U.S. tactical nuclear force, Nunn said "no small portion . . . are more dangerous to the alliance" than they are to the Warsaw Pact countries.

He said "two-thirds of NATO's 7,000 . . . warheads" have a range of fewer than 100 miles and most of those are nuclear artillery shells that travel fewer than 20 miles.

"Knowledge that the bulk of NATO tactical nuclear weapons, if employed, would be confined to strikes on NATO territory is hardly likely to terrify or deter the Soviet Union," Nunn said.

Nunn's remarks, in a little-noticed speech here Monday, have been widely reported in West German newspapers as a direct threat to the countries involved, according to Washington Post foreign correspondent Michael Getler.

The NATO alliance is expected to decide at a December ministerial meeting in Brusse's whether it will go ahead with the new medium-range missile systems.

They have become controversial because they would give the alliance, for the first time in more than 15 years, a nuclear system that could strike the Soviet Union from Western European bases.

The Soviet Union strongly opposes deployment of these missiles in Europe and is pressuring NATO to oppose them.

In a sharp attack, Nunn called "the Netherlands, Belgium and the left wing of the ruling Social Democratic Party in Germany" the weakest links in the NATO chain on this issue."

Opposition to the deployment of the new missiles has come from individuals in the countries mentioned by Nunn and has raised doubts whether the NATO ministers will make a positive decision in December.

Nunn is the leading congressional spokeman on NATO military matters with influence both among his colleagues and within the executive branch.

A White House spokesman said yesterday that there would be no comment on the speech, although Nunn's "opinion is respected."

Nunn argued forcefully that the older, short-range warheads should be replaced in Europe by the longerrange Pershing II and ground-launched cruise missile systems now being developed.

These systems, with ranges of over 1,000 miles, would permit NATO forces to hit targets in the western portion of the Soviet Union.

Currently, the longest range NATO missile system is the Pershing 1a with a 400-mile range. NATO fighter-bombers could deliver nuclear bombs on Warsaw Pact or even Soviet targets but, as Nunn pointed out, these systems are vulnerable to a first-strike attack even by conventional weapons.

According to Nunn, NATO today cannot "credibly threaten Soviet forces" in Russia nor "even Pact forces in certain parts of Eastern Europe . . ."

The new missiles, Nunn said, "by threatening destruction of Pact forces before they reach NATO soil, would reduce NATO's present heavy reliance on short-range weapons."

He added that if the allies went along with the new missile deployments, that step, along with other command and control improvements, "would open the way to reductions in the number of nuclear warheads deployed in Europe."

From the beginning, the Carter administration plan has linked production and deployment of the new missiles with an arms limitation offer to the Soviets and a cutback in short-range warheads now in the European nuclear stockpile.