The US-led force in Afghanistan has incorrectly reported a decline last year in Taliban attacks, officials said Tuesday, casting doubts on NATO's claims of progress in the war.

Officials said a reported seven percent decrease in "enemy-initiated attacks" for the whole of 2012 was wrong and insurgent attacks were roughly the same as the previous year.

The incorrect figure had been posted on the website of NATO's International Security Assistance Force (ISAF).

"This is a regrettable error in our database systems that was discovered during a routine quality check. We are making the appropriate adjustments," Pentagon spokesman George Little told news conference.

The US government and its allies have repeatedly touted a purported drop in insurgent attacks as proof that the Taliban is on the retreat. But the error in the key statistic raised questions about how governments and commanders are portraying the war effort.

But the Pentagon insisted that the error did not call into question its assessment that the Taliban had been weakened and pushed out of towns and cities.

"In spite of the stated adjustment, our assessment of the fundamentals of progress in Afghanistan remains positive," Little said.

Officials said the mistake occurred because some Afghan units have only recently taken the lead from NATO-led forces in some areas and had not entered numbers correctly into a database.

"The error is down to a large number of Afghan units coming online, a fact of their taking the lead in 2012 for the first time in a majority of districts," said a US defense official, who spoke on condition of anonymity.

The accurate number for insurgent attacks in 2012 showed the assaults had remained at the same level as in 2011, at more than 3,000, the official said.

"Overall, the numbers are about the same," the official told AFP.

Defense Secretary Leon Panetta, top military leaders and other senior officials often refer to promising security trends in Afghanistan and that violence and attacks are down from previous periods.

In a report to Congress issued in October, the Pentagon hailed what it called a drop in enemy attacks, with the decline registering in May and running through September 2012.

The admission of the error on Tuesday raised questions about the accuracy of the statistics cited by the Pentagon and NATO, particularly as the latest numbers are being compiled by Afghan troops and police with modest training and low literacy levels.

But the Pentagon said it appeared the mistake did not reflect a wider problem with statistics used to evaluate the war.

"I view this as a limited instance at this stage. If there is a broader problem, of course, we'll be forthright about it," Little added.

The Taliban have waged an 11-year insurgency against the Kabul government since being ousted from power in a US-led invasion in 2001.

The United States and NATO have around 100,000 troops in Afghanistan, but the vast majority of them will leave next year, with Afghan forces gradually taking over.

President Barack Obama announced earlier this month plans to withdraw 34,000 US troops from Afghanistan in the next year. The move effectively halves the size of the current 66,000-strong American force in Afghanistan.