Another wave of NATO bombardment today failed to persuade Gen. Ratko Mladic, the Bosnian Serb army commander, to withdraw about 300 tanks, mortars and other heavy weapons from around Sarajevo. The general's defiance immediately generated international rumors that a rift had emerged between Mladic and Serbian President Slobodan Milosevic, who is negotiating a peace settlement on all Serbs' behalf. But foreign affairs correspondent Marguerite Michaels reports that Mladic is acting in full concert with his patron. "The Serbs are doing something very interesting, which is to draw attention to the fact that the U.N. is not being neutral in this war. Serbs have long complained that the Muslims can go out on sorties against them and then retreat to safe areas to be protected by the U.N. So with this, the Serbs are saying, OK, let's negotiate, but don't call a club a heart here -- let's acknowledge that the U.N. is not being neutral."