KABUL: Amid fierce fighting after the Taliban captured the northern Afghan 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 in a recently declassified, heavily redacted Pentagon report that lays bare the confusion over rules of engagement governing the mission in Afghanistan.

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

In the heat of the battle, lines can be blurred, and the problem is not exclusive to Afghanistan: questions have arisen over the role of US troops in Iraq after a US Navy SEAL was killed by Islamic State 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 investigators in a report into 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, and 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, much of it blacked out for security reasons, 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."

US mission under review

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 say 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 told Reuters.

"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.

Until the end of 2014, when their combat role officially ended, NATO forces in Afghanistan peaked at more than 130,000 troops, most of them American. NATO's presence today is a fraction of the size.

Different operations can merge

Around 10,000 US troops are divided between the NATO train-and-assist mission called Resolute Support and a US-only counter-terrorism operation against militant groups that include al Qaeda and Islamic State but not the Taliban.

Under publicly declared rules of engagement, US advisors in Resolute Support generally cannot attack Taliban targets except in self defence.

As government forces have struggled, however, the definition of "self defence" has appeared less sharply defined, with some US air strikes conducted to defend partnered Afghan units.

The Kunduz report indicates at least some US troops have been sent into battle with questions unanswered.

The Green Beret complained that failure to provide clear guidance represented "moral cowardice", and that political leaders intentionally keep the mission vague.

That allows them to "reap the rewards of success without facing the responsibility of failure," he added.

Soldiers pleaded for "clearer guidance" and more clarification of overly complicated rules, according to investigators.

The Pentagon has not fully publicized rules governing the use of force by US troops, who may be called upon to act under either type of mission, sometimes in the same battle.

In the four days leading up to the hospital attack, US special forces called in nine close air support strikes under the authority of counter-terrorism, and 13 under Resolute Support, according to the report.

As part of self-defence, coalition troops have "some latitude" in calling air strikes on militant targets that may not be directly attacking them, but could soon pose a threat, Cleveland said.

Last year the Pentagon announced that Afghan forces could be helped under extreme conditions.

Additionally, under a "Person with Designated Special Status" classification, Afghan units operating closely with international advisers can be protected by air strikes as if they were coalition forces, according to Cleveland.

Who is the enemy?

Further complicating matters are counter-terrorism rules that allow strikes against al Qaeda, as well as militants linked to Islamic State 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 are working more closely together, signalling that the dominant Taliban group could once again be attacked by more air strikes.

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.

The second officer said he went into the Kunduz operation unsure of which authorities his unit would be operating under.

The lack of explicit instructions led the officer to choose his "default" of Resolute Support authorities, which he described as "just the safe bet."