BONN, Germany -- Bosnian Serb leader Radovan Karadzic's decision to agree to reopen Tuzla airport Tuesday came as a Serb offensive, long predicted by Western military analysts, showed signs of materializing in Bosnia.


The attempt by six Bosnian Serb warplanes to bomb a Moslem-controlled munitions factory in Novi Travnik, which resulted in four of them being shot down by NATO aircraft, was far from the only Serb offensive military action.


Serb artillery and infantry assaults against the enclave of Maglaj, combined with renewed shelling of Tuzla, suggested that a genuine spring offensive may already be underway.


But in sending aircraft from Banja Luka, the main airport held by the Bosnian Serbs, to Travnik, held by the mainly Moslem Bosnian government, the Serb military leadership may have been intent on goading the West into a response, said Western sources, who spoke on condition of anonymity.


This was the first major violation of the United Nations' no-fly zone since it was approved nearly one year ago, and NATO had no choice but to act under publicly stated U.N. policy, the diplomatic sources said. The Serbs seemed intent on drawing that response.


There was no Western response to three other examples of offensive actions in Maglaj, Tuzla and Sarajevo.


Sarajevo radio said Serb artillery pounded Maglaj, a town of 16,000 people said to be in a desperate state of deprivation, with more than 1,000 shells Sunday. U.N. sources and European Community monitors reported a Serb infantry offensive Monday against Maglaj, but there was no indication how much territory they may have gained.


Britain and France last week proposed declaring Maglaj a "safe zone" that would qualify for NATO air protection, but the plan was held up at the U.N. Security Council over objections by Russia and other problems, U.N. sources said. The Bosnian Serbs also continued a heavy artillery assault Sunday and Monday against the besieged city of Tuzla.


The Bosnian Serbs also managed to embarrass NATO and the United Nations on Monday when they drove a half dozen tanks they had hidden near Sarajevo out of the NATO-declared exclusion zone a week after they were to have turned them over to the United Nations