A Pentagon report has laid bare the confusion over rules of engagement governing the mission in Afghanistan.

Key points: A 700-page Pentagon report shows troops don't understand their role following 2014 withdrawal

A 700-page Pentagon report shows troops don't understand their role following 2014 withdrawal Critics say confusion stems from political narratives that define troops as "advisers"

Critics say confusion stems from political narratives that define troops as "advisers" US troops report the situation is more complex on the ground

The heavily redacted document has revealed that amid fierce fighting after the Taliban captured the northern city of Kunduz last year, US special forces advisers repeatedly asked their commanders how far they were allowed to go to help local troops retake the city.

They got no answer, according to witnesses interviewed for the review.

As the Taliban insurgency gathers strength, avoiding enemy fire has become increasingly difficult for US "advisers" who have been acting as consultants rather than combatants since NATO forces formally ceased fighting at the end of 2014.

Similar questions have also arisen over the role of US troops in Iraq after a US Navy SEAL was killed by Islamic State militants this month.

"'How far do you want to go?' is not a proper response to 'How far do you want us to go?'," one special forces member told those investigating the US air strikes on a hospital in Kunduz that killed 42 medical staff, patients and caretakers.

That incident was the biggest single tragedy of the brief capitulation of Kunduz to Taliban militants, although there is no suggestion that the mistake was the result of a lack of clarity over the rules of engagement.

But the 700-page report sheds light on how the rules are not fully understood, even by some troops on the ground, compromising the mission to stabilise the nation and defeat a worsening Islamist insurgency.

The issues exposed in the report are likely to be considered by the new US commander in Afghanistan, General John Nicholson, as he prepares to makes recommendations in the coming weeks that may clarify or expand the level of combat support the US-led training mission can provide.

"It's not a strategy and, in fact, it's a recipe for disaster in that kind of kinetic environment," said the soldier, who, like others in the report, was not identified.

He added that his unit, whose role was to advise and assist Afghan forces without engaging in combat, asked three times for commanders to clarify the rules governing their mission.

"Sadly, the only sounds audible were the sounds of crickets ... though those were hard to hear over the gunfire," the soldier said.

'Rules of engagement trapped in the jaws of politics'

Further complicating matters are counter-terrorism rules that allow strikes against Al Qaeda, as well as militants linked to the Islamic State group — which did not exist when the US military intervened in Afghanistan in 2001 — but not the Taliban.

In recent weeks US commanders in Afghanistan have reported that Al Qaeda and the Taliban were working more closely together, signalling that the dominant Taliban group could once again be attacked by more air strikes.

Barack Obama announced that US troops would stay in Afghanistan for most of 2016. ( Reuters: Jonathan Ernst, file )

Calling the authorities in Afghanistan "exceptionally complex", previous training had failed to prevent confusion, the Kunduz report found.

Prior to deploying to Afghanistan, commanders made clear that "combat operations was mostly a thing of the past", another special forces soldier said in the report.

On the ground, however, things were more complicated.

While acknowledging a lingering "lack of understanding in the West" about the US and NATO role in Afghanistan, US military spokesman Brigadier General Charles Cleveland denied there was confusion among troops over the broader mission.

More than 9,000 US soldiers were "retrained" on the rules of engagement following missteps in Kunduz, in an effort to reduce future misunderstandings, he said.

Critics said the confusion comes from political expediency, because US leaders are keen to portray the Afghan operation as designed mainly to help local forces fight for themselves.

"The rules of engagement are trapped in the jaws of political confusion about the mission," a senior Western official said.

"Nobody in Western capitals seems willing to admit that Afghanistan is a worsening war zone and ... that their troops are still battling out a combat mission on a daily basis," added the official, who declined to be named.

Reuters