Al Qaeda's Yemen-based wing threatened more small-scale attacks against the United States to inflict economic damage, particularly to the aviation industry.

Al Qaeda in the Arabian Peninsula (AQAP) said it had cost just $4,200 to mail two parcel bombs from Yemen to the United States last month. The bombs were intercepted in Britain and Dubai, sparking a worldwide security alert.

"It is such a good bargain for us to spread fear amongst the enemy and keep him on his toes in exchange of a few months of work and a few thousand bucks," AQAP said in its online Inspire magazine, released on militant websites.

"We are laying out for our enemies our plan in advance because as we stated earlier our objective is not maximum kill but to cause (damage) in the aviation industry, an industry that is so vital for trade and transportation between the US and Europe."

The United States had already stepped up airline passenger security after a Nigerian man tried to detonate explosives hidden in his underwear on a flight from Amsterdam to Detroit last December. AQAP had claimed responsibility.