It is now clear that new Soviet minisubmarines, the kind detected in Swedish waters last year, give the Kremlin new ability to spy on allied naval movements around United States and other coasts.
This is the view of military sources here, who draw implications from the new minisubs which patrol more widely than Sweden or even Scandinavia as a whole.
In their latest official report, the Swedes say that of the six Soviet submarines ''probably involved'' in incidents close to Stockholm last October, three were a new ''mini'' type, about 50 feet long, carrying two or three men, and traveling on caterpillar tracks on the seabed.
The Swedes photographed and studied track marks left on the bottom of the sea close inland to Stockholm itself.
While the Swedes now say they will sink any more alien intruders, and Denmark and Norway have heightened their own vigilance as a result, military sources see many advantages to the Soviets of such tiny subs in even more sensitive waters elsewhere.
The minisubmarines could be transported on larger Soviet submarines to stations off the two US coasts and into the Mediterranean and Aegean seas as well.
The small craft could then slip along the seabed into shallower areas not normally patrolled by naval defenses, in effect outflanking antisubmarine surveillance. They would, in theory, pick up information on the types of naval craft passing near them. They could also bring back vital hydrographical data, such as the strength and direction of ocean currents and bands of salinity in the sea.
''Submarines can use these salinity bands to hide beneath,'' says one expert here. ''The salt has the effect of deflecting sonar defenses.''
Experts say minisubmarines could clearly be useful to the Soviets just off the Norfolk naval base in Virginia, where the US training fleet is based, and off the west coast where new US Ohio-class submarines are being built. By being on the sea floor, the subs could also detect and possibly neutralize electronic sensing devices planted by coastal defenses.
''We've known for some time that the Soviets have small submarines for salvage and rescue work,'' says one London-based military analyst. ''They are carried on the India-class submarine.
''It is a logical extension for the Soviets to develop seabed 'mini' types for surveillance. What's new in the developments in Sweden is the evidence that the Soviets are now using the small craft to get close in-shore.''
Experts here are not clear on how the new minisubs are powered - whether the tracks themselves transport the craft or whether propellers on the stern do the work. If propellers are used, as is suspected, and the tracks give traction and height above mud and debris, then experts see the new craft as extremely useful indeed.
Meanwhile, the Swedish, Norwegian, and Danish governments are all hardening their attitudes toward the Soviet Union.
* In Stockholm, the government of Prime Minister Olof Palme has sent a stiff protest note, has decided to keep its recalled ambassador from Moscow home for a week so that he will miss May Day in Red Square, and has spoken in language unusually tough for a neutral country. The government report, prepared by a special commission, said 40 incidents ''of an increasingly provocative'' nature had involved alien craft off Sweden in 1982 alone.
* In Norway, the Navy searched for what may have been a submarine sighted near a big naval base on the west coast near Husnes. NATO's northern fleet is scheduled to hold maneuvers in the region soon. The Norwegians have also expelled a Soviet research vessel from Oslo harbor for entering without a permit.
* The Danish Defense Ministry has ordered a new look at Danish coastal defenses because of the Swedish report. Danish coastal waters are too shallow for submarines, but could be vulnerable to the new minisubs. The ruling Danish Democratic Party has also protested by postponing indefinitely a visit to Moscow by a four-man delegation to have been headed by former Foreign Minister Kjeld Olesen.
Meanwhile, the Soviet news agency Tass has denied Swedish assertions that Soviet submarines penetrated Swedish waters.