ABOARD THE J.R.M. KOSMAJ (AP) _ A faint white sliver is all that can be seen of the NATO frigate through thermal imaging binoculars from the bridge of the Yugoslav navy patrol boat.

''She is at the edge of our territorial waters, but we can relax as long as she doesn't cross the line,'' said Lt. Vuceta Stanisic, commander of the Kosmaj, as he gazed into the night. Nearby, a Yugoslav missile boat prowled the Adriatic off the strategic Bay of Kotor.

''They are there for support if we should get into any serious trouble,'' said Stanisis, noting his vessel's single 40mm anti-aircraft gun would be inadequate in a long-range ship-to-ship engagement.

''We know the navy would bear the brunt of (a NATO) attack, but I doubt they would use missiles against an insignificant patrol boat - and we can outrun everything else they've got,'' chuckled Stanisic, 25, as waves crashed over the prow of the 96-foot, 150 ton boat.

Tensions were running high in the Adriatic after the U.N. Security Council voted Thursday to sanction the use of force to ensure humanitarian supplies reach war-torn Bosnia.

A flotilla of about a dozen NATO and Western European Union frigates and destroyers are stationed in the southern Adriatic. They are watching for cargo ships violating trade sanctions imposed by the United Nations in May against Serbia and Montenegro - the remainder of federal Yugoslavia blamed for inciting civil war in Bosnia.

Late Friday, the U.S. Navy ordered the aircraft carrier Saratoga and its 80 warplanes to move into the Adriatic closer to Bosnia, Defense Department sources said. The move places the Saratoga and its accompanying ship, the cruiser USS Biddle in position to support aid shipments.

Many in Serbia fear the U.N. resolution could pave the way to air and naval strikes on military targets in their country if Western troops escorting humanitarian convoys in Bosnia clash with Serb rebels there.

''I am, of course, concerned about an eventual attack. But this navy has no other coast to withdraw to, and there can be no dilemma that we will defend it to the very end,'' said navy Cmdr. Adm. Nikola Ercegovic.

''If the intention of the West is to provide humanitarian aid to people in Bosnia, there is no reason to attack Yugoslavia, much less our fleet,'' he said. ''But our country has been demonized to such an extent abroad, that it is difficult to predict what their real intention is.''

The feared confrontation comes at a particularly trying time for the navy, which has been transformed from a formidable blue-water force into little more than a coastal defense patrol as Yugoslavia disintegrated.

Unlike the army and air force, the navy managed to withdraw the bulk of its units unscathed from the war with Croatia - where its largest bases and repair yards were located.

But it lost about 40 percent of its officers and men - including all but three of the 16 admirals - when Slovenia and Croatia seceded last year. The navy now has less than 8,000 sailors and marines, a force insufficient to man all of its ships.

Now, with a coastline of only 60 miles long - barely one-tenth its pre- breakup territory - the navy is searching for a new role for its four missile frigates, five attack and six midget submarines, 30 torpedo and missile boats and 30 patrol boats.