Serbs invading from Croatia have advanced around six miles almost to the centre of the Bihac pocket and, with renegade Muslim militiamen attacking from the north, have gobbled up 30 square miles in less than a week.
Bosnian Serbs joined the onslaught on Sunday with artillery attacks from the south and east that may presage a tank-backed infantry assault aimed at slicing the enclave in two, isolating Bihac town in the south.
"This co-ordinated, deliberate attack on all fronts represents arguably the most considerable military action in Bosnia for many months," said Lieutenant-Colonel Chris Vernon, a UN military spokesman.
Alexander Ivanko, civilian spokesman for the UN mission in Bosnia, described the invasion as an "extraordinarily escalatory step".
"Such a flagrant violation of an international border is becoming a trademark of the Croatian Serbs whose disregard for international law is unprecedented," Mr Ivanko added.
The Bihac crisis could explode into a wider war as Croatian government troops are poised along UN truce lines near the Adriatic to attack the Krajina Serbs if Bihac looks like falling.
A Serb conquest of Bihac, a pocket sitting between self-proclaimed Serb states in Bosnia and Croatia, could dash Zagreb's ambition to overwhelm its own Serb rebels.
Bihac bestrides the most direct road and rail links between breakaway Serb domains in Croatia and Bosnia.
Bosnian Croat forces, allied to the Croatian army, have recently ploughed north along the mountain border with Croatia and have squeezed Krajina Serb supply lines.