The US military will no longer divulge facts and figures about its costly effort to assist Afghan security forces, declaring the information top secret, officials said Thursday.

The move marks an about-face for the Pentagon, which for the past six years has reported a range of data about the $65 billion program to build up the Afghan National Security Forces (ANSF).

The information included how US taxpayers' money has been spent and the state of the troubled country's police and army.

John Sopko, special inspector general for Afghanistan reconstruction (SIGAR), voiced disappointment with the change and said the classification of such a volume of information was "unprecedented."

"The decision leaves SIGAR unable to publicly report on most of the $65 billion US-taxpayer-funded efforts to build, train, equip and sustain the ANSF," Sopko's office said in its latest report.

The US commander in Afghanistan, General John Campbell, defended the move, saying the information -- which includes an assessment of the combat readiness of Afghan units -- could prove helpful to Taliban insurgents and needed to be kept secret.

"With lives literally on the line, I am sure you can join me in recognizing that we must be careful to avoid providing sensitive information to those that threaten our forces and Afghan forces, particularly information that can be used by such opposing forces to sharpen their attacks," Campbell wrote to the inspector general in a letter dated January 18.

Data about the assistance program has provided a key measure for gauging the effect of the US-led effort to bolster the Afghan forces, particularly as some of the details have exposed the shortcomings of the Afghan military.

The inspector general's regular updates on the state of the program have highlighted the Afghan force's problems with "attrition," as large numbers of recruits have died on the battlefield, deserted or quit.

The decision to withhold facts about the effort raises questions about the condition of the Afghan security forces after the withdrawal of most NATO troops last year.

A small, mostly American contingent of about 12,000 troops remains on the ground to advise and assist the Afghans.

Lawmakers criticized the secrecy, saying it undermined attempts to hold the government accountable for massive reconstruction projects.

"I'm offended that this previously unclassified information is now being classified, in a move deeply detrimental to our efforts to root out waste and fraud," said Senator Claire McCaskill, a member of the Senate Armed Services Committee.

- 'No answers' -

The New York Times condemned the move on its editorial page, saying "it strains credulity to believe that insurgents would become more proficient fighters by poring over lengthy inspector general reports about an increasingly forgotten war."

The newspaper, which first reported the change, added that keeping the information secret "unreasonably prevents American taxpayers from drawing informed conclusions about the returns on a $107.5 billion reconstruction investment that, adjusted for inflation, has surpassed the price tag of the Marshall Plan."

According to the SIGAR report, the military gave classified or restricted answers to more than 140 questions posed by the inspector general.

This included the total amount of US funding for Afghan forces for the current year, details of contracts for literacy training and an assessment of anti-corruption initiatives.

The State Department also was not forthcoming about its aid projects when contacted by the inspector general's office, the report said.

Despite a legal obligation for federal agencies to provide requested information to the inspector general, "the State Department did not answer any of SIGAR's questions on economic and social-development this quarter, and failed to respond to SIGAR's attempts to follow up."

But the State Department said Thursday there was no intention to withhold information and that officials had told the inspector general the delivery of the latest data would be delayed.

Congress has approved $107.5 billion for reconstruction in Afghanistan, with $15.3 billion yet to be disbursed.