Japan's decision to buy the US-built F-35 fighter jet gives a shot-in-the-arm to a troubled program faced with a litany of technical setbacks and a ballooning budget, analysts say.

Japan's announcement of the $4.7 billion deal was welcomed at the Pentagon where officials have defended the program's progress despite fresh revelations of engineering flaws.

By opting for the Joint Strike Fighter, Japan hopes to bolster its aging air force to counter China's growing military power while also nurturing its security ties with the United States that have been strained by a dispute over an American air base in Okinawa, said Doug Bandow, a senior fellow at the CATO Institute.

"I suspect the purchase reflects a combination of perceived military value, alliance relations, and a message to China that was seen as outweighing the cost and any problems," Bandow told AFP.

Apart from Lockheed Martin's F-35, Japan considered two other jets -- the Boeing-made F/A-18 Super Hornet and the Eurofighter Typhoon.

Bandow said it would have been "a stretch" for Japan to opt for a European warplane as Tokyo's strategic alliance with the United States is so deeply entrenched and it has long relied on American defense firms for military hardware.

The move, however, is a gamble that the aircraft will not be bogged down by engineering problems, cost overruns and incessant delays.

A Pentagon spokesman portrayed Japan's decision as a vote of confidence in the radar-evading plane that can carry missiles, precision-guided bombs and a 25mm Gatling cannon.

"We also appreciate their confidence in our industry and in our government teams. We look forward to working with them," Captain John Kirby told reporters.

Japan joins 11 other countries in the F-35 project. Lockheed Martin and the US government view global partners, including eight nations sharing program costs, as crucial to bringing down the overall price of the Joint Strike Fighter.

But defense officials have faced an uphill battle to contain the cost of the program, which has swelled to $385 billion.

Technical problems have forced retrofitting efforts and a slower pace of production, driving up the price of each aircraft to roughly $113 million.

A report by five Pentagon experts that leaked last week revealed a total of 13 engineering problems, including five issues deemed of major concern, and suggested an ambitious production schedule would have to be revised.

The report cited the results of initial flight tests and warned that more demanding tests had yet to be carried out.

"This airplane is far from over its hard times," said Winslow Wheeler of the Center for Defense Information, an outspoken critic of the program.

Speculation is growing that Defense Secretary Leon Panetta will call for scaling back production to save money when he unveils the proposed Pentagon budget for 2013.

For several US allies invested in the program, the F-35 has become a contentious political issue. According to Wheeler, Japan will soon have buyer's remorse as well.

"The remorse is going to start setting in in a month or two once Panetta announces whatever production reductions he's going to announce," he said.

"It's unavoidable that they announce delays and reductions for the 2013 budget for the F-35," he said.

Japan plans on buying its first four aircraft in 2016 at a firm price and then will purchase "as many as they can afford," Wheeler said.

"They clearly understand that the price is going through the roof."

F-35 advocates insist the program is on the right track and that over time, the engineering difficulties -- which include flaws in the helmet mounted display and landing gear for the carrier version -- will be resolved while the cost of the aircraft will steadily decline.

"Despite the discovery of numerous small problems in testing, all three versions of the F-35 are making steady progress," said Loren Thompson, chief operating officer of Lexington Institute.

"The most recent Pentagon report on the program's status found no fundamental design issues that threatened aircraft production," said Thompson, whose think tank receives funds from Lockheed Martin and other defense contractors.

The F-35 offers Japan an advanced aircraft it needs for the 21st century, he said.

"Japan wanted an aircraft that China was unlikely to match or counter effectively over the next three decades, so that dictated a stealthy fighter with advanced sensors and datalinks," he said.

Current US plans call for three variants -- including a model for aircraft carriers and a vertical-landing version -- for a total of 2,456 jets for the Air Force, Marine Corps, and Navy.