"On behalf of all of US Forces - Afghanistan, today's loss is heartbreaking and we offer our deepest condolences to the families and friends of our service members who lost their lives today," said John Nicholson, the top US and NATO commander in Afghanistan.
"Despite today's tragic event, we are steadfast in our commitment to help our Afghan partners defend their nation."
The killings come just days before the US presidential election .
The US military, which leads a NATO mission to train and assist the Afghans, often gives upbeat assessments about Afghan military performance.
But as Afghan military forces near the end of a second year leading security operations across their battered country, they are being killed in horrific numbers.
Afghan police and army units in 2015 succeeded NATO in providing security for Afghanistan. That year an estimated 5,000 were killed and another 15,000 wounded.
The death toll this year is much worse already, with 5,523 Afghan troops killed in the period from January 1 to August 19, according to the US watchdog SIGAR.