The acknowledgment came after the US government's office of the Special Inspector General for Afghanistan Reconstruction (SIGAR) published a report saying the US military had barred it from disclosing how much of Afghanistan is under Taliban control.

Such a restriction would have represented a significant break from past accountability amid mounting security woes in the war-torn nation.

But after the report came out, Navy Captain Tom Gresback, a spokesman for NATO's Resolute Support mission in Afghanistan, said the intent was not to withhold or classify information which was available in prior reports.

"A human error in labeling occurred," Gresback said in a statement.

"The classification system, because it incorporates both a NATO and US nomenclature, can be challenging, and a mistake was made. The data is not classified and there was no intent to withhold it unnecessarily."

At issue are the number of Afghan districts, and the populations living in them, considered to be held or influenced by the Kabul government, by insurgents, or contested by both.

The US government has sometimes referred to such numbers in the 16-year-old war to show how the Afghan security forces are faring against a resurgent Taliban.

The mistaken move to classify the data comes after Washington last year did agree to an Afghan request to classify the number of Afghan security forces killed or wounded in the conflict.

The special inspector, John Sopko, wrote in SIGAR's latest quarterly report that the request to classify more information was "troubling."

SIGAR said the Pentagon had also asked its office, for the first time since 2009, to classify figures detailing the size and attrition rates of Afghan security forces.

Resolute Support said that as of October 2017, approximately 56 percent of Afghanistan's 407 districts were under Afghan government control or influence, 30 percent were being contested by the Taliban or other groups, and approximately 14 percent were under insurgent control or influence.

- Cause for concern -

General John Nicholson, who commands US and NATO forces in Afghanistan, has said 80 percent of the Afghan population could be under government control within about two years, up from less than two-thirds today.

But tracking progress toward such a goal would be difficult without any numbers being released.

"Historically, the number of districts controlled or influenced by the government has been falling since SIGAR began reporting on it, while the number controlled or influenced by the insurgents has been rising," Sopko said.

Militants including the Taliban and the Islamic State group have stepped up their attacks on beleaguered Afghan troops and police in recent months, sapping morale already hit by desertions and corruption.

On Saturday, a Taliban suicide attacker driving an explosives-packed ambulance blew it up in a crowded area of the capital, killing at least 103 people -- mainly civilians -- and wounding 235 in one of the worst bombings in Kabul in recent years.

On January 20, Taliban fighters stormed the city's landmark Intercontinental hotel and killed at least 25 people, the majority of them foreigners, in an assault lasting more than 12 hours.

And on Monday, gunmen and suicide bombers launched a pre-dawn attack on a military compound in Kabul, killing at least 11 soldiers and wounding 16. The attack was claimed by IS.