The quarterly report from the Special Inspector General for Afghanistan Reconstruction (SIGAR), released in April, had incorrectly stated that Afghan security forces had been depleted by about 10 percent over the course of a year.

Based on that figure, there were estimated to be 296,409 active military, police and intelligence personnel as of January 31.

But on Tuesday SIGAR said corrected figures from United States Forces-Afghanistan "indicate a total ANDSF (Afghan National Defence and Security Forces) strength of 313,728 as of Jan. 31".

"The new numbers still show that overall ANDSF strength declined sharply from January 2017 to January 2018 (by 17,980 personnel), though not as sharply" as reported, the watchdog said.

The confusion was "the latest in a series of problems SIGAR" has faced regarding information about Afghan forces, it said.

The updated figures come on the heels of another deadly day in Afghanistan as the Taliban launched an assault on the western city of Farah.

Commandos from Herat and Kandahar were rushed to the provincial capital as US and Afghan air forces carried out airstrikes. This helped to push the insurgents to the outskirts of Farah by early Wednesday, though a clearing operation was still going on.