Banja Luka, the largest city in Bosnian Serb hands, is said to be of such strategic and economic importance that Serbia's President, Slobodan Milosevic, could not permit its fall.
However, the fears of Mr Rifkind and other Western officials may be exaggerated. All the evidence since last May, when the Serbs in Croatia and Bosnia began to suffer their string of stunning defeats, suggests that Mr Milosevic is keen to remain out of the war.
He did nothing to save the Croatian Serb enclave of western Slavonia when the Croats overran it in early May. He did nothing to save Krajina when it fell to the Croats in early August, although that defeat resulted in the flight of 150,000 Serbs and the collapse of a centuries- old Serb civilisation.
Mr Milosevic has done nothing to prevent the Croatian army, the Bosnian Croats and the Muslim-led Bosnian government forces from taking such large tracts of territory from the Bosnian Serbs that they now appear to control more than half of Bosnia. These losses have resulted in an exodus of almost 100,000 Serbs, but Mr Milosevic appears unperturbed.
After Nato began its aerial offensive against the Bosnian Serbs on 30 August, the Serbian leader confined himself to requests that the military campaign should stop. In fact, during a period in which his Croatian and Bosnian Serb brethren have suffered some of the most severe humiliations in Serb history, Mr Milosevic has stayed well out of the way, preferring to negotiate a peace deal with US envoys than to charge into the war.
Diplomats and Serbian sources in Belgrade suggest that the one thing that might trigger Serbian military action is a Croat attempt to recapture eastern Slavonia, the last Serb-held region of Croatia.
Significantly, when Richard Holbrooke, the US negotiator, announced a diplomatic breakthrough in peace talks at Geneva on 8 September, he pointed out that no agreement had been reached on eastern Slavonia. The Serbian leader has been pressing for eastern Slavonia to be put under some form of international supervision for a period of five years or so. This has the advantage, from Mr Milosevic's point of view, of delaying the restoration of Croatia's authority over the region and allowing a breathing-space in which an opportunity might arise to assert Serbia's control.
Knowing this, Croatia is insisting that a transitional period for eastern Slavonia should last only a year, and that there must be no obfuscation of the central point that the region belongs to Croatia. Serb procrastination, the Croats warn, will merely result in the recapture of eastern Slavonia by force.
Mr Milosevic's nationalist critics in Serbia have rained abuse on him for his supposed betrayal of the Serbs, but with his tight control of the media, police force and bureaucracy in Serbia, he has suppressed domestic opposition.
The last real challenge to his authority was mounted inMarch 1991, when tens of thousands of demonstrators massed in Belgrade and Mr Milosevic was forced to bring in the tanks. This year he has treated his foes with contempt, imprisoning one of his main nationalist rivals, Vojislav Seselj, and paying no attention at all to the fragmented liberal opposition.
Of course, the fall of Krajina and the displacement of almost 250,000 Serbs in less than two months was not what Mr Milosevic had in mind when he launched his bid in 1990-91 to build a state incorporating all Serbs wherever they lived. But now it seems that a peace settlement is of more value to him than the vain pursuit of the Greater Serbian dream.
Partly, this is because it will ensure the lifting of UN sanctions, although Serbian propagandists and many Western commentators have exaggerated the damage caused to the Serbian economy. The most important sanction is the denial of hard-currency credits, without which the renewal of Serbian industry will be impossible.
Another important reason for Mr Milosevic's interest in peace is that he expects it to end Serbia's status as the outcast of Europe. As the largest of the Yugoslav successor states, Serbia could play a powerful regional role in the future, but it needs to lose the tag of international pariah.
Ultimately, Mr Milosevic's main concern is to stay in power. He has achieved this goal at enormous cost - above all, the collapse of historic Serb lands in Croatia - but he has never lost sight of his objective.Reuse content