The decision was slammed on Monday by the US government's Special Inspector General for Afghanistan Reconstruction (SIGAR) John Sopko, who said it would hamper the watchdog's ability to chart progress among US-funded Afghan troops.
"USFOR-A said the casualty data belonged to the Afghan government, and the government had requested that it be classified," Sopko said in the latest quarterly report documenting the use of US funds in Afghanistan.
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There was no immediate comment from US Forces or Afghan officials.
Sopko said the move "will hinder SIGAR's ability to publicly report on progress or failure in a key reconstruction sector", noting that more than 60 percent of the roughly $121 billion in US funding spent since 2002 has gone on Afghan security forces.
Casualties among Afghan forces in the early part of this year were "shockingly high" as they struggled to beat back the resurgent Taliban and other militant groups, SIGAR has said previously.
They soared by 35 percent in 2016, with 6,800 soldiers and police killed, according to the watchdog.
As of February only about 60 percent of Afghanistan's 407 districts were reported to be under government control, SIGAR has said.
Sopko said the decision was only the second time that the American military has sought to classify data on the Afghan security forces' capabilities.
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The first time was for the January 2015 quarterly report, which came on the heels of the withdrawal of US-led NATO combat troops from Afghanistan at the end of 2014.
During the most recent quarter, Sopko travelled to Afghanistan where he held a series of meetings with Afghan President Ashraf Ghani, Chief Executive Abdullah Abdullah and other senior officials.
"On each occasion, President Ghani noted the National Unity Government's close cooperation with SIGAR and thanked us for our help fighting corruption," Sopko said.
Ghani also "vowed to account for every penny of American assistance to his government".
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The SIGAR report comes as the US sends more than 3,000 additional troops to Afghanistan - on top of the 11,000 already on the ground - to train and assist Afghan security forces as part of a new strategy outlined by US President Donald Trump in August.
The Taliban has been rampant in recent years and Trump's open-ended committment to put more American boots on the ground and a surge in US airstrikes has further fuelled the insurgency.
But Afghan security forces have been struggling to beat back the insurgents amid desertions, killings and corruption.
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