The Afghan Taliban launched their “spring offensive” Friday, heralding fresh fighting in the drawn-out conflict as embattled security forces struggle to recover from a devastating attack on a military base one week ago.

Operation Mansouri — named after the group's former leader, who was killed in a US drone strike in May 2016 — will target foreign forces with tactics including “conventional attacks, guerrilla warfare, complex martyrdom attacks, insider attacks”, a statement from the insurgents said.

The annual spring offensive normally marks the start of the “fighting season”, though this winter the Taliban continued to battle government forces, most successfully in last week's attack on the military base outside the northern city of Mazar-i-Sharif.

The assault last Friday, in which militants dressed in Afghan army uniforms and with valid passes to the installation slaughtered at least 135 young recruits, is believed to be the deadliest by the Taliban on an Afghan military target.

It sparked widespread anger and led to the resignations of the defence minister and the army chief of staff, along with a reshuffle of army corps commander, leaving security forces facing disarray as fear and suspicion grew that the militants had inside help.

Authorities have arrested at least 35 soldiers over the incident so far, ranked from foot soldier to colonel.

So-called insider attacks — when Afghan soldiers and police turn their guns on their colleagues or on international troops — have been a major problem during the war, which began in 2001.

Afghan forces saw soaring casualties in 2016, up by 35 per cent with 6,800 soldiers and police killed, according to a US watchdog.

Already beset by killings, desertions, and struggles over leadership and morale, they have been straining to beat back insurgents since US-led Nato troops ended their combat mission in December 2014.

With more than one third of Afghanistan outside of government control, civilians continue to bear a heavy brunt, with thousands killed and wounded each year.

A UN report for the first quarter of 2017 showed children are paying an increasingly disproportionate price, with 210 killed from January to March, up 17 per cent from the same period last year.

'Another tough year'

The Taliban announcement comes days after Pentagon chief Jim Mattis visited Kabul as the Trump administration seeks to craft a new strategy in Afghanistan.

Mattis warned that 2017 would be “another tough year” for Afghan security forces, but would not be drawn on recent calls by the US commander of Nato forces in the country, General John Nicholson for “a few thousand” more troops to break the insurgency.

The Afghan conflict is the longest in US history — US-led Nato troops have been at war there since 2001, after the ousting of the Taliban regime for refusing to hand over Osama bin Laden following the 9/11 attacks in the United States.

The US has around 8,400 troops in the country with about another 5,000 from Nato allies, as efforts to negotiate a lasting peace settlement between Kabul and the Taliban have repeatedly fallen through.

They are largely conducting a training, advise and assist mission aimed at supporting Afghan forces.

Western instructors, likely German and American, were on the 30,000-strong base outside Mazar when it was attacked, but were not involved in repelling the assault, security sources have told AFP.

Earlier this month, the American military dropped its largest non-nuclear bomb on the militant Islamic State (IS) group hideouts in eastern Afghanistan, killing nearly a hundred militants, according to unverified figures from Afghan officials.

The bombing, the first time the GBU-43/B Massive Ordnance Air Blast — dubbed the “Mother of All Bombs” — had been used in combat, triggered global shockwaves.

But it was criticised by observers who questioned its use against a group that is not considered as big a threat as the resurgent Taliban.

Two US troops were killed Wednesday and one was wounded while fighting IS militants near where the bomb was dropped, the Pentagon has said.