The US Justice Department is challenging a proposed merger between US Airways and American Airlines. The $11bn deal would create the world's largest airline, but there are concerns it would also reduce domestic competition and push up airfares. If US Airways and American Airlines get together, almost ninety percent of all US air travel will be in the hands of just four big companies. Al Jazeera's John Terrett reports from New York. For more on this story click here: http://aje.me/13VSyco