The nightmare vision of advanced Russian submarines prowling silently through the world's oceans and sinking warships at will should have been laid to rest at the end of the Cold War.

But Western officials are increasingly alarmed by the huge sums Moscow is pouring into new attack submarines intended to play a much more aggressive role than in the past."We find the Russian submarine investment worrisome, the fact that a country asking us for aid money is pouring billions into submarines," said Rear Adm. James Stark, president of the U.S. Naval War College.

"Where is the threat to them coming from? It is not coming from the United States, Britain or Norway. Their requirements for naval forces are around their own coasts, where there is a lot of instability," he said in an interview.

"Therefore I don't see the need for them to have a high-technology building program."

Experts say the improved Russian attack submarines, designed to roam the oceans in the hunt for large vessels and other submarines, are quieter than their Western counterparts and have more ef-fec-tive weapons.

Rupert Pengelley of Jane's International Defense Review magazine said although some Western commanders were talking up Russian superiority in a bid to boost defense budgets, there was no doubt Moscow had stolen a march on its competitors.

"The Russians have advanced at a faster rate than was predicted. They have outdone themselves," he said.

"They could afford to have noisier submarines because they don't need to get as close to their targets as Western craft, but in fact they are quieter," he said.

"The Russians are also changing up a gear and deploying their submarines much further away from home."

Pengelley said Moscow wanted submarines capable of operating at long distances because it assumed that future surface warships would be able to launch cruise missiles against Russian land targets from as far away as 2,800 miles.

To ram home the point, Russian nuclear attack submarines have twice carried out long-term deployments near U.S. fleets in the last year. Pengelley said U.S. observers had been very impressed by the submarines' performance.

"What they've had as a priority is extending their defense against very long-range missiles. They want a shield against the U.S. systems," said John Berryman, a Russian strategic studies expert at the University of Wolverhampton.

"What would happen if Sweden and Finland joined NATO? It could be explosive. The perception is that the United States is now putting into effect a new policy of containment," he said.

Western intelligence officials say that although Moscow has just 99 submarines today compared with 186 a decade ago, more than half of the existing craft can slip unnoticed into the West's main sea lanes.

"I have great admiration for what the Russians are doing with their submarines," said a senior British naval official, who asked not to be identified.

"The thing which worries me is not the Russians' naval capabilities but the general air of instability around NATO's borders. What is going to happen in the future?"

Talk of spending money on new underwater craft jars with the sight of countless rusting Russian ships tied up at port. But Moscow seems to have decided that submarines offer better value than expensive and vulnerable surface ves-sels.

"They have lost a lot of units . . . but those which have gone are the older, less capable ones, the liabilities," said Sir Julian Oswald, former chief of the British naval staff.

"Noise reduction means their submarines can operate far further out and recent forays into the North Atlantic deliver a political message. The force is undoubtedly leaner and fitter," he told a recent conference on the Russian navy.