DUBAI (Reuters) - Al Qaeda’s Yemen-based wing vowed to “bleed” U.S. resources with inexpensive, small-scale attacks that cost militants just thousands of dollars to mount but billions for the West to guard against.

A worker checks a parcel at Sanaa International Airport November 5, 2010. REUTERS/Khaled Abdullah

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

It singled out the aviation industry as its main target.

“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, posted overnight 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 U.S. and Europe.”

American Admiral Mike Mullen, who as chairman of the Joint Chiefs of Staff is the top U.S. military officer, called AQAP a serious threat to the United States on Sunday. He added it has become substantially more dangerous over the past two years.

“This branch of al Qaeda is very lethal and I believe them -- in terms of what they say they’re trying to do (to attack the United States),” Mullen told CNN television’s “State of the Union” program.

The United States 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.

It also hiked counterterrorism assistance to Yemen to $155 million in fiscal year 2010, from just $4.6 million in 2006. U.S. officials are also looking at additional ways to put pressure on militants, including enhanced training of Yemeni forces.

STRATEGY OF A THOUSAND CUTS

“This strategy of attacking the enemy with smaller, but more frequent operations is what some may refer to as the strategy of a thousand cuts,” Inspire magazine said, according to a translation by Ben Venzke, an expert on militant publications and CEO of IntelCenter.

“The aim is to bleed the enemy to death.”

AQAP said last month’s failed parcel bomb operation, where the bomb-loaded printers had been sent from Yemen’s capital, Sana, to two synagogues in Chicago, had been cheap to execute.

“Two Nokia mobiles, $150 each, two HP printers, $300 each, plus shipping, transportation and other miscellaneous expenses add up to a total bill of $4,200,” AQAP said.

“We will continue with similar operations and we do not mind at all in this stage if they are intercepted.”

“To bring down America we need not strike big,” it added.

Soon after the discovery of the explosive printers, AQAP had also claimed responsibility for the crash of a UPS cargo plane in Dubai in September, but U.S. officials have said that there were no indications that the parcel delivery company’s plane had been brought down by an attack.