The report, released by the Special Inspector General for Afghanistan Reconstruction (Sigar), said that the number of security forces personnel decreased by about 10 per cent to just under 300,000 in the past 12 months.
The findings come a day after a wave of attacks in Afghanistan killed 41 people.
However, the Afghan Defence Ministry rejected the findings, telling the BBC that the Army had enough soldiers to fight the militants.
"We have enough soldiers, enough Afghan National Army units to fight the militants and other mafia groups. We reject the Sigar report on numbers. It's not 100 per cent fact," a Ministry spokesman told the BBC.
"The morale of our soldiers is very high and as of now there are 11 on going operations across the country."
The report said more of the population was falling under the control of militant groups including the Taliban.
It said the Taliban and other militant groups control or influence 14.5 per cent of Afghanistan's 407 districts -- the highest level since the group started recording such data in late 2015.
According to an earlier BBC study, Taliban fighters are openly active in nearly three-quarters of the country. The research also suggested that the Islamic State (IS) militant group is more active in Afghanistan than ever before, although it remains far less powerful than the Taliban.
On Monday, twin attacks claimed by the IS in the capital left 30 people dead, including nine journalists and photographers. One BBC reporter was shot dead in a separate attack in Khost province.
In August 2017, with the Taliban gaining ground and the security situation deteriorating, US President Donald Trump announced that the US military would stay in the country indefinitely. The Pentagon later said it would send 3,000 extra troops.
More than 8,000 US special forces remain in the country supporting Afghan troops, although NATO combat operations officially ended in 2014.
(This story has not been edited by Social News XYZ staff and is auto-generated from a syndicated feed.)
