NATO'S widening strikes on Bosnian Serb positions are turning the Bosnian conflict into a face-off between NATO and Bosnian Serb military commander Ratko Mladic. NATO and Bosnian Serb forces engaged in their fiercest long-range clashes ever Sunday in the wake of failed secret talks between Bosnian Serb military leader Ratko Mladic and senior UN officials. General Mladic continues to refuse to pull back his heavy weapons from around besieged Sarajevo, but NATO's southern commander, US Adm. Leighton Smith, seems determined to bring Mladic to his knees. ''This has turned into a very, very personal confrontation between two men,'' says a senior UN official. ''Ratko Mladic and Leighton Smith.'' But far more is at stake than Mladic's and Smith's pride. For the Clinton administration, a high-stakes gamble of using airstrikes to back up a new peace initiative in the Balkans could be dragging it into what appears to be open warfare with the Bosnian Serbs. With an election year ahead, a Bosnian Serb triumph would undermine Clinton's image as a leader abroad - or at least not give him the foreign policy clout his administration had hoped for. For the Bosnian Serbs, a withdrawal of heavy weapons from around Sarajevo would be the ultimate capitulation to Western pressure and remove their most powerful pieces of leverage over the Bosnian government - the strangulation of Sarajevo. And for NATO, the credibility and prestige of the world's most powerful fighting force is on the verge of being shattered by one of the world's smallest but most determined armies. If NATO cannot cow the Bosnian Serbs, the cold-war-era alliance's effort to turn itself into the United Nation's new no-nonsense enforcer will have been dealt a fatal blow, and NATO's long-term existence called into question. Talks don't work Sunday's fast-moving clashes and slow-moving meetings indicated that the Bosnian Serb determination may be hardening as Western resolve is softening. Mladic again stated his refusal to withdraw his heavy weapons from around Sarajevo at a meeting on Saturday with Russian First Deputy Foreign Minister Igor Ivanov in Belgrade. And early on Sunday, Bosnian Serb forces fired more than 80 shells at the UN-controlled airport in the UN ''safe area'' of Tuzla. UN commanders quickly called in NATO close-air support, and at least two planes attacked the source of the firing. After the Tuzla clashes, French President Jacques Chirac abruptly announced a pause in NATO airstrikes for talks between Mladic and UN Gen. Bernard Janvier. The talks went poorly, according to NATO officials. Mladic repeated his earlier offer to open the Sarajevo airport and roads into the city, but again said he could not remove all 300 of his heavy weapons from around the city because it would leave Serb-held suburbs of Sarajevo open to Bosnian government attack. But Bosnian President Alija Izetbegovic, apparently under Western pressure, guaranteed that his government troops would not launch an offensive in the Sarajevo area if Serbs withdrew their heavy weapons. Shortly after the meeting ended, General Janvier and NATO southern commander Smith approved the firing of 13 cruise missiles at air-defense installations around Banja Luka, the largest Serb-held city in northern Bosnia. The preemptive strike, which was backed by Maverick missiles fired from airborne F-16 fighters, was designed to take out the sophisticated air-defense system that shot down US pilot Capt. Scott O'Grady in June. ''It was time to take out those air defenses in northwest Bosnia,'' said Capt. Jim Mitchell, spokesman for NATO southern command in Naples. ''They were dangerous to us.'' The targeting of the system is a clear attempt to unnerve the Serbs and send a clear message that all their territory, not just their holdings around Sarajevo, are now naked to NATO attack. But the dramatic escalation in NATO firepower risks triggering a massive Bosnian Serb response and could turn Europeans against continued bombing. Some UN officials are openly questioning the effectiveness of the bombing, and it is unclear whether Mr. Chirac acted with US approval. ''There are very, very strong indications that they are not going to move their heavy weapons. What is the use of continuing? How long do you go on until you become a combatant in the mind of the Serbs?'' said a UN official. Milosevic's role A crucial sign that Mladic may be able to hold out also emerged Sunday. Powerful Serbian President Slobodan Milosevic attended Mladic's meeting with Russia's Mr. Ivanov. Mr. Milosevic is the only man believed to be powerful enough to force Mladic to withdraw his weapons from around Sarajevo. The Western powers are also relying on him to bring peace to the region. But Milosevic's presence at a meeting where Mladic refused to move the weapons indicates Milosevic's tacit approval of Mladic's defiance. It also indicates that should a peace deal be signed that involves the Bosnian Serbs giving up territory to the Muslim-led government, Milosevic is unlikely to force a defiant Mladic to actually give up the territory. Only a week and a half after bombing began, the Bosnian Serbs also appear to be yet again driving a wedge between the US and its European allies. A European tendency to back away from use of force and to stay neutral in the conflict is clashing with US calls for a massive use of force against the Bosnia Serbs.