A day after Bosnian Serb leader Radovan Karadzic stunned the world by making concessions to reduce hostilities mainly towards United Nations troops, reports from the former Yugoslavia suggest that his forces aren't completely living up to their leader's promise. If Karadzic keeps his word, former president Jimmy Carter -- yesterday summoned by Karadzic to mediate the conflict -- is certain to be on his way to Bosnia as early as this weekend. But today aid workers were detained by the Serbs, a British helicopter on a U.N. transport mission was fired on and Bihac was again attacked. As a result, the fate of Carter's mission seemed uncertain. Karadzic has said the unilateral moves -- which include a ceasefire around Sarajevo and eased transportation for U.N. and other neutral parties -- would be implemented within 24 hours. Carter will be going as a private citizen -- not as a representative of the Clinton Administration, White House press secretary Dee Dee Myers said today. That's because Bosnian Serbs have regularly made and broken similar promises. At this point, few countries trust their word.