A US government watchdog is pressing the Pentagon to explain reports of tens of thousands of "ghost" soldiers and police on the payrolls of the Afghan security forces, which are heavily funded by international donors.

The US government has allocated more than $68 billion since 2002 to help support Afghan security forces battling Taliban insurgents and other militants. The United States and its NATO allies pledged earlier this year to provide around $5 billion per year until at least 2020 for the army and police.

Some of that money could be fraudulently wasted by funding non-existent positions in the security forces, John Sopko, the Special Inspector General for Afghanistan Reconstruction, wrote in a letter to the U.S. Defense Department.

The letter was sent in August but released publicly on Friday.

"Persistent reports indicating discrepancies between the assigned force strength of the (Afghan security forces) and the actual number of personnel serving raise questions regarding whether the US government is taking adequate steps to prevent taxpayer funds from being spent on so-called 'ghost' soldiers," Sopko wrote.

Afghan forces are struggling to defend against Taliban militants seeking to reimpose a fundamentalist Islamist government in Afghanistan, as well as other militant groups.

In northern Afghanistan, government troops have been battling since Monday to try to clear Taliban fighters from positions they seized in key city of Kunduz.

Nationally, the Afghan army and police have an approved strength of about 320,000, but officials say the real number is much lower than that.

Heavy casualties and soldiers and police deserting or not reporting for duty mean security forces lose thousands of personnel every month, which they struggle to replace.

The shortage of personnel has been most acutely felt in hot spots like Helmand province in southwest Afghanistan, where a lack of troops has undermined efforts to blunt Taliban gains.

Officials in Helmand have said as many as half the security forces on the rolls did not exist, with much of the salaries for the non-existent troops going to corrupt leaders, Sopko said, citing media reports.

The US Department of Defense has taken steps to try to prevent fraud by automating some systems and collecting biometric data to track police and soldiers, but Sopko said such measures would only be effective if accurate data on Afghan force levels was collected and maintained.