WASHINGTON — More than 16 years after the U.S. helped overthrow the Taliban in Afghanistan, the metrics kept by U.S. and Afghan officials and security experts show the Taliban is gaining territory and strength.

As Kabul reels from a deadly wave of terror attacks, the numbers tell the tale. The percentage of the Afghan population under the control of the central government has slipped, the land mass under the control of coalition forces is shrinking, and the number of Taliban fighters may have doubled in the past four years.

In 2014, U.S. officials told NBC News that the number of Taliban fighters in Afghanistan was about 20,000. Four years later, one U.S. defense official said the current Taliban strength is at least 60,000. Another senior U.S. official said 60,000 "passes the sniff test," while a third official said 60,000 is "a place to start.

An Afghan official told NBC News earlier this month that the Afghan estimate of Taliban strength is also 60,000. That marks a significant increase from the estimate of 35,000 that Afghanistan's TOLOnews attributed to an Afghan defense official in 2011.

The U.S. military does not release official numbers on how many Taliban are in Afghanistan. One U.S official called such estimates a "fool's errand" because the fighters often change their allegiance from one terror group to another.

"It's a wildly varying planning figure," the official said, explaining the U.S. military needs a marker to plan to fight but is hopeful many fighters are not ideological and will eventually lay down their arms and "find a reason to identify with Afghanistan nationalism and the larger good."

Part of the reason for the apparent increase in Taliban strength is integration between the Taliban and a separate group of Islamist militants, the Haqqani network. According to the Pentagon's June 2017 Enhancing Security and Stability in Afghanistan report, "Haqqani and Taliban integration has become so robust that many observers no longer look at them as separate entities, but as factions within the same group."

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Bill Roggio, senior fellow at the Foundation for the Defense of Democracies and editor of the Long War Journal, says that 60,000 is his low-end estimate of Taliban strength, "given the amount of territory the Taliban control and contest, and the level of fighting in Afghanistan even in the winter."

Roggio tracks how much territory the Taliban controls in Afghanistan, and reports that its control of land has grown each of the past 2 years.

Roggio says that the Taliban currently controls about 45 of the country's 398 districts, and is battling for control of 117. That is up from 82 districts that it controlled or contested in 2016, by his calculation, and 70 in 2015.

While Resolute Support, the U.S.-led military command in Kabul, also tracks Taliban territorial control, it stopped releasing that information to the public in 2015.

According to the Special Inspector General for Afghanistan (SIGAR), Resolute Support asked SIGAR not to make public the U.S. military's most recent internal estimates of Taliban control.

The appendix to SIGAR's January 2018 quarterly report, released Tuesday, says that Resolute Support asked SIGAR not to release either its estimate of Taliban territorial control or its estimate of the percentage of the Afghanistan population under Taliban control. The appendix quoted Resolute Support as saying the figures are "not releasable to the public." Both metrics are unclassified.

"This is the first time SIGAR has been specifically instructed not to release information marked 'unclassified' to the American taxpayer," said SIGAR in a cover letter to the report.

Capt. Tom Gresback, a spokesperson for Resolute Support, denied that information about population control was ever meant to be kept from the public, and said the conflict with SIGAR was a misunderstanding based on a "clerical error."

He provided NBC News with both of the numbers SIGAR had said it was asked to conceal.

According to Gresback, Resolute Support estimates that the percentage of the Afghan population under government control was 60 percent as of October 17, a significant decline from earlier estimates. The Afghan government controlled 70 percent in September 2016 and 65 percent in February 2017, according to previously published U.S. figures.

Gresback also said that as of October 2017 about 56 percent of the country's districts were under Afghan government control or influence, 30 percent were contested, and approximately 14 percent were under insurgent control. He didn't provide earlier figures of district control for comparison.

Despite these figures, a U.S. official said there is "no indication at all that the Taliban are growing or strengthening."

"They are not getting any stronger," the official said, adding that the Taliban are moving fighters around the battlefield to maintain control of certain areas.

In its October 2017 quarterly report, SIGAR said the U.S. military had restricted its release of data by classifying previously releasable information. The military had classified data about the casualties, strength and readiness of the Afghan military and police. All numbers showed that the capabilities of the Afghan uniformed services were deteriorating.

The U.S. has more than 14,000 troops in Afghanistan.