ZAGREB, Croatia (AP) _ U.N. efforts to prevent fighting from spreading to a tense Serb-held corridor in northern Bosnia could actually help cement Serb war gains.

Since the corridor is the link that connects Serb gains in Bosnia and Croatia with Serbia proper, some analysts believe the U.N. move may be decisive for the outcome of the entire Yugoslav conflict.

Over the weekend, the United Nations deployed its first military observers in the corridor, located around Brcko near the Bosnian-Croatian border. That followed a warning by the U.N. Security Council last week to all parties - including the Bosnian government - not to start trouble in the region.

''If this U.N. trend continues, the Serbs will practically get the international community's protection for their vital corridor,'' said Fran Visnar, a military analyst in Zagreb.

But Matthew Nerzig, U.N. spokesman in Zagreb, said its action are ''only intended to prevent hostilities now.''

Both sides have been massing troops and weaponry in the area, which is perceived as crucial for future developments in Bosnia and Croatia.

Without it, Serbs who rebelled and captured large swaths of northern and western Bosnia, and southern and central Croatia, are cut off and vulnerable. Serb nationalists' dream of a Greater Serbia would be frustrated.

With it, Serbia can continue to support and coordinate policy with those Serbs in Bosnia and Croatia.

They have held the Brcko corridor almost since the beginning of the Bosnian war in 1992. In order to emphasize their hold on Brcko, the Bosnian Serb assembly plans to meet there later this week.

''Brcko and the corridor are the key to the Serb state across the Drina River, and we are not going to give them up,'' Momcilo Krajisnik, Bosnian Serb assembly speaker, said.

In the two-year-old Bosnian conflict, the United Nations has designated six Muslim enclaves as ''safe areas'' in an attempt to protect them from the Serbs.

But the Brcko area poses a different problem because the town and some surrounding areas are Serb-held, while the area north is Croat-held and Muslims control small chunks of territory to the south. Therefore, the area has become a new potential flash point in the Bosnian conflict.

But quelling fighting and inserting troops there could just cement the status quo.

''Preserving the status quo in the Serb-held corridor clearly suits the Serbs,'' said Neven Kulenovic, a spokesman at the Bosnian embassy in Zagreb. ''Though the Serbs would certainly like to widen the corridor, their prime concern is to preserve it.''

The strip of land is between three and six miles wide.

At Serb-held Brcko, once mostly Muslim, the corridor shrinks to its narrowest, squeezed between a small Bosnian Croat wedge of Orasje in the north and joint Muslim-Croat forces in the south.

Allies in the 1992 war against Bosnian Serbs, Muslims and Bosnian Croats fell out in mid-1993 over territories in central Bosnia. Their enmity ended with the signing of an agreement in March to form a federation in Bosnia with ties to Croatia.

Though the joint military command envisaged has yet to take shape, Croats and Muslims have cooperated throughout the war in northern Bosnia.

''We are fighting for Bosnia,'' said Djuro Matuzovic, commander of the Bosnian Croat forces in Orasje, in a recent TV interview. ''Our only solution is to be together with the Muslims.''

All sides appear ready for a fight. Government forces gathered about 40,000 men on the southern side of the corridor. Bosnian Serbs have less men but are estimated to be twice as strong in heavy weaponry.

''He who wins there, has won the war,'' said Visnar.