Russian forces in Chechnya silenced their heavy artillery and stopped their bombing raids Monday following President Boris Yeltsin's declaration of a cease-fire as the first step in a new peace plan for the republic, but casualties continued to mount regardless. Twenty-eight Russian soldiers were killed after the start of the cease-fire at midnight Sunday, when their column was ambushed near Vedeno in southeast Chechnya, Interfax reported. Sixty-nine were wounded. But if the rebel fighters were not yet observing Yeltsin's unilaterally declared cease-fire, Russian artillery, which has for three weeks been pounding separatist rebels dug in at Bamut, did hold its fire, Interfax said. Yeltsin had announced the midnight deadline in a televised speech Sunday evening, in which he laid out his long-awaited plan for bringing an end to the 16-month-old war in Chechnya.