KABUL (AFP/Reuters ) - The Taliban in Afghanistan vowed on Saturday to start a new campaign of mass suicide attacks on foreign military bases and diplomatic areas, as well as damaging “insider attacks”, as part of a new spring offensive this year.

The offensive was announced via emails from Taliban spokesmen.

The Taliban statement said this year’s offensive, named after Khalid bin Waleed will involve ‘special military tactics’ similar to those carried out previously.

“Collective martyrdom operations on bases of foreign invaders, their diplomatic centres and military airbases will be even further structured while every possible tactic will be utilized in order to detain or inflict heavy casualties on the foreign transgressors,” the statement said.

The spring offensive was coordinated to begin on April 28 to coincide with a national holiday to mark the overthrow of the Soviet-backed government of Mohammad Najibullah in 1992, the statement said.

The insurgents’ latest statement celebrated the start of the Nato withdrawal, saying that “the enemy, with all its military might, has been overwhelmed and finally forced to flee from their military bases”.

It added that this year’s offensive would start on Sunday “in unison throughout the country... against the transgressing invaders and their degenerate backers”.

It warned Afghans working for President Hamid Karzai’s ‘stooge’ regime to distance themselves from the government to avoid being caught up in the promised violence, and called for young people not to join the police or army.

However, there was no immediate reaction to the Taliban’s statement from the Nato-led International Security Assistance Force (Isaf).

The group has made similar announcements in recent years, which have sometimes been followed by spikes in violence after Afghanistan’s harsh winter months.

The announcement of more mass suicide and insider attacks will likely be greeted with concern by the Nato-led military coalition, which is in the final stages of a fight against the Taliban-led insurgency that began in late 2001.

After announcing their spring offensive last year, the Taliban launched a large attack in Kabul involving suicide bombers and an 18-hour firefight targeting Western embassies, Isaf headquarters and the Afghan parliament.

The start of the traditional ‘fighting season’ is particularly important this year, with Isaf increasing the rate at which it hands security responsibility to Afghan forces before the withdrawal of most foreign troops by the end of 2014.

Insider attacks, also known as “green on blue” attacks, involve Afghan police or soldiers turning their guns on their Isaf trainers and counterparts. They have grown considerably since last year and have strained relations between Kabul and foreign forces.

However, there is considerable debate over how many can be attributed to infiltration by insurgents and how many are by disgruntled members of the Afghan security forces.

Last August, then Isaf commander, US General John Allen, said about a quarter of such attacks involved the Taliban. Nato combat operations in Afghanistan are due to end next year, and coalition commanders say that the local army and police have made enough progress to provide security and keep the Taliban at bay.

Meanwhile, a Nato plane crashed in Afghanistan on Saturday killing four service members, the coalition said, adding that early reports suggested the aircraft was not brought down by insurgents.

The plane crashed in the south of the country, according to a statement from Nato’s International Security Assistance Force (Isaf). "The cause of the crash is under investigation, however initial reporting indicates there was no enemy activity in the area at the time of the crash," it said.Most international soldiers in southern Afghanistan are from the United States, but the nationality of the dead was not released in line with coalition policy.

There was no immediate claim of responsibility from the Taliban, who earlier Saturday announced the start of their annual "spring offensive" vowing a nationwide series of attacks as Nato troops withdraw from Afghanistan.

Aircraft crashes are fairly frequent in Afghanistan, where the 100,000-strong international mission relies heavily on air transport as it battles the insurgency across the south and east of the country. Earlier this month, two US troops were killed when a Nato helicopter crashed in the eastern Afghanistan, while five US troops died in the southern province of Kandahar in May when their helicopter came down during a heavy rainstorm.