The US state of Hawaii has tested a nuclear attack warning siren for the first time since the end of the Cold War. <br/><br/>The resumption of the monthly tests comes amid a growing threat from North Korea's missile and nuclear programme. <br/><br/>Hawaii, in the Pacific, already has a monthly test of sirens warning of natural disasters, including tsunamis. <br/><br/>The nuclear attack signal uses a different, wavering tone, warning residents and tourists to stay indoors and await further instructions.