WASHINGTON - US President Barack Obama and British Prime Minister David Cameron vowed a united front on Wednesday against the threat posed by Islamist extremists and their 'distorted ideology.' In a joint opinion article that appeared in Thursday's Times of London newspaper and released on Wednesday night in Washington, Obama and Cameron outlined the principles of the US-British alliance ahead of two days of White House meetings. The two leaders are to have a working dinner on Thursday night and hold more talks on Friday.