An audiotape purportedly by one of America's most-wanted insurgents in Iraq, Abu-Musab al-Zarqawi, that was posted Friday threatens more attacks against US forces there and warns against collaborating with Americans. The authenticity of the 18-minute tape, posted on a Web site, could not be determined. The voice on the tape directly addressed President Bush. "You, Bush, we will not rest until we avenge our dignity," the voice said. "We will not rest while your army is here as long as there is a pulse in our veins." The tape urged al-Zarqawi's followers to step up their attacks on American soldiers, vowing to "make swords drip with their blood."