The government is to review an air defense system established after the September 11, 2001 attacks to determine whether the costly program is still necessary, the New York Times reported Friday.

The review is expected to include a complete reassessment of the threat of a terror attack by air and will consider the cumulative effects of various security measures taken since the 2001 attacks.

"The fighter force is extremely expensive, so you always have to ask yourself the question 'How much is enough?'" Major General Pierre Forgues, the Canadian who currently serves as director of operations for the North American Aerospace Defense Command, commonly known as Norad, told the paper.

The Times notes that US "Gen. Victor E. Renuart Jr., the head of Norad and Northern Command, ordered the review in response to criticism this year by the Government Accountability Office that the command had failed to conduct 'routine risk assessments.' General Renuart expanded the scope of the review 'to refine how we allocate and request resources today,' said the commands’ spokesman, James Graybeal."

The program currently involves keeping dozens of military aircraft and hundreds of air crew on alert to respond to any threats, though air patrols of US cities ended in 2007.

The review of the program, expected to be complete by next spring, will look at the likelihood of terrorists hijacking a plane or piloting their own aircraft into US or Canadian airspace.

It will try to assess whether the air defense program is necessary in light of various security measures, including airport screening, passenger tracking and secured airliner cockpits, that were implemented after the 9/11 attacks.

"The ability of terrorists to do what they did on 9/11 has been greatly curtailed," Forgues told the Times.

However, he warned that the outcome of the review was not predetermined and the program could be maintained at current levels or even scaled up if deemed necessary.

Norad statistics show that there were 1,000 incidents of suspicious air activity last year, with 400 requiring a Norad response and 200 prompting jet fighters to be scrambled, the Times said.