NEW : Islamic State kill troops , mostly in southern Yemen . Fiercest clashes in Taiz since March began , witnesses say . A government official says Yemeni President Ali Abdullah Saleh is ` ` severely hurt '' Opposition leaders express cautious support for the vice president .
NEW : Islamic militants kill troops , civilians in southern Yemen . Fiercest clashes in Taiz since demonstrations began , witnesses say . A U.S. official says Yemeni President Ali Abdullah Saleh is `` severely hurt '' Opposition leaders express cautious support for the vice president .