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A federal watchdog overseeing U.S. efforts in Afghanistan slammed the Defense Department on Monday night for blocking the release of unclassified data on U.S. progress there, calling the order unprecedented and "troubling for a number of reasons."

In a letter accompanying its regular quarterly report, the office of the Special Inspector General for Afghanistan Reconstruction, or SIGAR, said the Defense Department blocked publication of data on "the number of districts, and the population living in them, controlled or influenced by the Afghan government or by the insurgents, or contested by both."

Those data aren't classified, but the Defense Department determined that "they are not releasable to the public," said the letter, which gave no indication that the Defense Department provided a reason for the decision.

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The letter, which is signed by Inspector General John Sopko, called the order distressing because "the number of districts controlled or influenced by the Afghan government had been one of the last remaining publicly available indicators for members of Congress — many of whose staff do not have access to the classified annexes to SIGAR reports — and for the American public of how the 16-year-long U.S. effort to secure Afghanistan is faring."

On a broader scale, the directive is troubling because "this is the first time SIGAR has been specifically instructed not to release information marked 'unclassified' to the American taxpayer," the letter said.

In its last report, in October (PDF), SIGAR said the Taliban controlled or was contesting 43 percent of Afghanistan's districts, up from 40 percent in July.

Afghan soldiers stand guard at Marshal Fahim military academy after an attack Sunday in Kabul, Afghanistan. Omar Sobhani / Reuters

"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 — a fact that should cause even more concern about its disappearance from public disclosure and discussion," SIGAR said Monday night.

In a statement to Reuters, the Defense Department said it wasn't responsible for the order, saying it had been issued by Operation Resolute Support, the NATO-led coalition in Afghanistan.

Related: U.S. has thousands more troops in Afghanistan than the Pentagon admits

The commander of Operation Resolute Support, however, is a U.S. Army general, John Nicholson Jr. In November, Nicholson told NBC News that the war in Afghanistan remains in a "stalemate" but that "we've set all the conditions to win."

The SIGAR complaint comes as Taliban and Islamic State fighters have increased their attacks in Afghanistan, especially in Kabul, the capital. Hundreds of people have been killed in a wave of attacks in the last few months.

On Jan. 20, at least 14 foreigners and four Afghans were killed when gunmen stormed the Intercontinental Hotel in Kabul, and at least five people were killed and two dozen injured when four gunmen stormed an office of the Save the Children aid agency in Jalalabad on Jan. 24.

On Saturday, a Taliban attacker drove an ambulance filled with explosives into the heart of Kabul, killing at least 103 people. Then, on Sunday, 11 Afghan troops were killed when ISIS militants attacked a military academy in Kabul.