Sweden's efforts to capture a foreign submarine thought to be trapped underwater in a military zone seem no closer to success now than six days ago when the search began, the naval officer directing the hunt conceded today.

"We are under pressure to do something spectacular, and the conditions are very difficult," said Vice Adm. Bror Steffenson, chief of the defense staff. As to whether the hunt might ever succeed, Steffenson said softly, "In my heart I'm very uncertain."

The admiral's comments tonight to reporters gathered in a naval facility here to monitor the massive sea and air search conveyed the deepening sense of frustration among the Swedish military over progress in the operation. Tuesday's exuberant predictions from spokesmen that the vessel would be forced to surface have now become subdued evaluations of the limitation of antisubmarine equipment and pleas for public patience.

"We lack the means to do what the government wants us to do," Steffenson said. Although sonar contact with the vessel was made twice during the day and five additional depth charges were dropped in the vicinity, the admiral raised the possibility that the intruder had evaded underwater blockades and escaped.

While the search operation -- estimated to be costing Sweden as much as $500,000 a day -- continues at full scale, the mood is now one of extreme caution. "Obviously something we are doing is not adequate," said another naval officer, "but what, we don't know."

The expectations of early success in capturing what spokesmen still believe is probably some kind of Soviet submarine arose from several factors. When the vessel was spotted last Friday it was so close to the country's most sensitive naval installation that resources were quickly mobilized to block its movement.

Commanders were given wide latitude to use explosives and experimental devices that damage but do not destroy submarines in the major air and sea search that was mounted. Within an enclosed area it was evidently assumed that the vessel could not hold out indefinitely.

The prospect of cornering a Soviet intruder -- only the latest of many such suspected spy missions -- caused a sensation among Sweden's press and public in which the military was swept along. "Somehow we've got to quiet this down," one spokesman said privately this morning, and Steffenson's statements later in the day were plainly the result.

Visiting the scene of the search on Horsfjarden Bay, about 25 miles south of Stockholm, it is easy to understand the difficulties the Swedes are facing. Although the area is not especially large -- about 12 miles long and 2 1/2 miles wide -- it is big enough for a small craft to keep moving away from searchers.

Helicopters circling overhead, a fleet of coastal patrol boats, a frigate, Swedish submarines and divers apparently may be able to spot the vessel but are unable to move quickly enough to catch it. Unless the submarine can be surrounded or damaged enough with depth charges to make it surface, the hunt could go on for days, the admiral acknowledged.

If the intruder did escape today, the maneuver was accomplished in mid-afternoon. Steffenson said a contact was made in the "neighborhood" of one of the two barriers erected with nets, chains and other equipment. But he declined to say on which side the contact was made.

There apparently was another contact with a vessel later in the day, which could mean the escape attempt failed. Some military and diplomatic sources, who stress they do not know for certain, say it could also mean that the submarine in the restricted area is actually a decoy or an unmanned spy craft of some new type that is being directed by another larger submarine farther out to sea.

Aside from raising the possibility that the submarine had escaped, Steffenson also said tonight that there may be more than one vessel involved. But he said little is known and, for security reasons, even less can be said publicly.

A Swedish newspaper reported late tonight that helicopters had dropped depth charges near a second submarine spotted outside the blockaded waters, The Associated Press reported.

Steffenson said that submarine hunting specialists from other countries consider the Swedish coastal area particularly difficult for a search because the terrain and the composition of water provide good cover for an intruder. "They have told us this is the most difficult place" to sustain an operation, he asserted.

The coordinated approach of today's efforts to dampen anticipation of early success suggests a shift in public relations policy among the senior defense staff. But beyond that, any real evaluation of the prospects of progress is virtually impossible.

Western diplomats who generally are kept fully informed by the Swedes on matters such as this say they have found their sources increasingly reticent. "It is possible they are preparing the people for disappointment," said one diplomat noting that after the buildup the letdown would be a big one.