KABUL - 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, killed in a US drone strike in 2016 - will target foreign forces with "conventional attacks, guerrilla warfare, complex martyrdom attacks, insider attacks", an insurgent statement said.

"The enemy will be targeted, harassed, killed or captured until they abandon their last posts," it continued.

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 massacre last Friday saw insurgents armed with guns and suicide bombs slaughter at least 135 young recruits, according to the official toll, though multiple sources have claimed it is much higher.

The assault is believed to be the deadliest by the Taliban on an Afghan military target since they were driven from power in 2001, and fuelled fears of insider attacks - when Afghan soldiers and police turn their guns on colleagues or on international troops.

The attackers carried valid passes to the base, security sources said, and were dressed in Afghan army uniforms. The defence minister and army chief have resigned, and at least 35 soldiers have been arrested over the incident.

Already beset by killings, desertions, and struggles over leadership and morale, Afghan forces have been straining to beat back insurgents since US-led NATO troops ended their combat mission in December 2014. They faced soaring casualties in 2016, up by 35 percent with 6,800 soldiers and police killed, according to a US watchdog.

With more than one third of Afghanistan outside of government control, civilians also continue to bear a heavy brunt, with thousands killed and wounded each year with children paying an increasingly disproportionate price, according to UN figures.

Afghanistan's Interior Ministry shrugged off the Taliban threats Friday, saying the offensive was "not something new". "We will target, kill, defeat and suppress the Taliban... all across the country," acting ministry spokesman Najib Danish told AFP. The Taliban statement said they would focus on state-building and "establishing mechanisms for social justice and development" in the areas under their control.

Afghan and international officials have repeatedly called on the Taliban to disarm and join the political process, a call they have so far refused. Kabul-based analyst Ahmad Saeedi said the insurgents - "emboldened" by government failures - would instead seek more territory this year.

"I believe this will be a difficult year for Afghan security forces, as they will be facing the resilient Taliban's complex and sophisticated attacks countrywide," he told AFP.

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 also warned that 2017 would be "another tough year" for Afghan security forces, but would not be drawn on recent calls by NATO commander in Afghanistan General John Nicholson for "a few thousand" more troops.

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

The bombing triggered global shockwaves, but was criticised by observers who questioned its use against a group that is not considered as big a threat as the Taliban.

Some analysts even argued the strike could boost the Taliban, who had been fighting a turf war with IS in Nangarhar province near the border with Pakistan, where the bomb was dropped.

Two US troops were killed Wednesday while fighting IS militants near the blast-site, the Pentagon has said, highlighting the price of its continuing role in the conflict.

Reuters adds: According to US estimates, Afghan security forces, which have suffered thousands of casualties, control less than 60 percent of the country, although the insurgents have so far been unable to capture any major provincial centres.

In February, the top US commander in Afghanistan, Gen John Nicholson said he needed a few thousand more international troops to break a "stalemate" with the Taliban.

The US currently has around 8,400 troops in Afghanistan as part of the NATO-led Resolute Support training and advisory mission, as well as a separate counterterrorism mission that mainly targets Islamic State and al Qaeda fighters.

The costs of that mission have been highlighted this month by the deaths of three US service members in operations against Islamic State along the border with Pakistan.

US officials say a solution to the conflict can only come through a political settlement which includes the Taliban, who have rejected peace talks while foreign forces remain in Afghanistan.

But there is as yet no clarity on Washington's broader strategic objectives for the region, including its approach to nuclear-armed Pakistan, which has long been accused of aiding the insurgents, a charge Islamabad denies.