The United States expressed concern Monday over North Korea\'s \'bellicose rhetoric\' after Pyonyang said it had nullified the armistice that ended the Korean War. \'We are certainly concerned by North Korea\'s bellicose rhetoric, and the threats that they have been making follow a pattern designed to raise tension and intimidate others,\' White House spokesman Jay Carney said. \'The DPRK will achieve nothing by threats or provocations, which will only further isolate North Korea and undermine international efforts to ensure peace and stability in north-east Asia.\'