BRUSSELS -- Bosnia's rebel Serbs are likely to keep the upper hand on the battlefield against the Moslems for some time, despite isolation from Serbia and an increasing flow of arms to government forces from outside, analysts say.


The Moslem-led government has taken the offensive against the Serbs in the hopes of taking back conquered territory, buoyed by international sympathy for their cause and the alliance with their erstwhile Croat enemies.


They may be disappointed.


Some short-term gains against the Serbs, who hold more than 70 percent of Bosnia and are well-equipped with heavy weapons from the former Yugoslav army, are likely.


But the Moslems badly need more heavy weapons, especially tanks and artillery, if they are to take and hold ground.


Previous gains by the Moslems have often been repulsed by Serb forces who moved in more big guns.


"Overall, I think the view is that the military balance in Bosnia will not really change for some time," said Colonel Andrew Duncan of the London-based International Institute for Strategic Studies. "The Moslems are still deficient in heavy weapons."


The Bosnian army numbers around 110,000, with 40 tanks and 400 artillery pieces. The Serbs have 80,000 men under arms, 330 tanks and 800 artillery pieces.


The Serbs have made attempts to boost their infantry force by drafting more able-bodied men.


The Bosnian army cannot rely on much support from Bosnian Croat troops, said by the institute to number 75,000.


But analysts say two elements could change the military balance in the longer term -- probably not before next spring.


If Serbian President Slobodan Milosevic keeps his pledge to cut all political and economic links with the Bosnian Serbs as punishment for their refusal to accept an international peace plan, the Serb forces could face a lack of supplies.


For now, NATO sources say they have adequate stockpiles of fuel, ammunition and spare parts. But, over time, they might face trouble in moving heavy weapons around.


"Modern armies work on fuel. Without fuel you've just got lumps of metal," the UN commander in Bosnia, British Lieutenant-General Sir Michael Rose, said earlier this week.


The second element would be the lifting of the UN arms embargo, which the United States has said it wants before the end of October if the Serbs continue to reject the peace plan.


But, even if that happened, it would take several months to ship in Western equipment and train the government forces to use it.


By that time winter would have set in, a difficult time for offensive action in Bosnia's hilly and wooded terrain.


The Moslems are already getting more weapons from breaches of the UN embargo, with the route through Croatia now open, but not the heavy weapons the Moslems need most of all.


There is also the question of morale.


The Moslem-led government feels that time now may be on its side while the Serbs are increasingly isolated politically.


"That could work either way," said one Western military official. "Either it will stiffen the Serbs' resolve and persecution complex, or it could undermine morale. We'll see."