KABUL - The Afghan Taliban announced Tuesday the start of their "spring offensive" even as the government in Kabul tries to bring the insurgents back to the negotiating table to end their drawn-out conflict.

The Taliban said in a statement they would "employ large-scale attacks on enemy positions across the country" during the offensive they have dubbed "Operation Omari" in honour of the movement's late founder Mullah Omar, whose death was announced last year.

The annual spring offensive normally marks the start of the "fighting season", though this winter the lull was shorter and they continued to battle government forces albeit with less intensity.

The statement promised "martyrdom-seeking and tactical attacks against enemy strongholds", a reference to suicide bombings - a strategy the group has long resorted to against its enemies, the Afghan police and army, which they view as "stooges" of the West. On Monday, 12 fresh recruits were killed in one such attack in the country's east.

The Islamists, who have been waging an insurgency since being toppled from power in 2001, also promised attacks on the 13,000 NATO troops currently stationed in the country, officially in a training and advisory role since the end of their combat mission in 2014.

"By employing such a multifaceted strategy it is hoped that the foreign enemy will be demoralised and forced to evict our nation," they said.

The Taliban have made the departure of all foreign forces a precondition to the resumption of direct peace talks with Kabul which began last summer in Pakistan but ended abruptly after it was revealed that Mullah Omar had been dead for two years.

Responding to the announcement Sediq Sediqqi, a spokesman for Afghanistan's interior ministry said: "The Taliban just want to show that they are still there. In the past 14 years they were not able to reach their goal and we will not allow them to do that" he said

Dawlat Waziri, a spokesman for the war-ravaged country's defence ministry, said that the government forces were prepared to hit back: "Now that the Taliban have rejected peace talks, we are prepared to respond to war with war."

At least five members of Afghanistan's border police force were killed in an insider attack in the southern province of Kandahar close to the frontier with Pakistan when a colleague opened fire on them before fleeing, officials said on Tuesday.

"Late last night, a border policeman opened fire on his colleagues and killed five of them. He crossed the border and went to Pakistan," Sameem Khapalwak, a spokesman for the provincial governor, said by telephone.

He said an investigation had begun into the incident, which occurred in the Wesh area of Spin Boldak district not far from Afghanistan's porous southern border with Pakistan.

A four-country group comprising Afghanistan, the United States, China and Pakistan has been holding meetings since January aimed at jump-starting negotiations, though their efforts have so far been in vain.

Mullah Omar's successor Mullah Akhtar Mansour, meanwhile, has won a string of impressive victories on the battlefield, helping to silence emerging factions by stepping up the intensity of his military campaign.

Last year the Taliban were able to briefly capture Kunduz, the first time they had held an Afghan city since the fall of their government in 2001. It is not clear whether the announcement of the spring offensive will lead to an immediate escalation in fighting.

Afghanistan has actively courted the NATO-led coalition to delay a planned drawdown of their troops stationed in the country, most of which are US, and maintain its air power and military support.

The Taliban's resurgence has raised serious questions about Afghan forces capacity to hold their own, with an estimated 5,000 troops killed last year, the worst ever toll.

Kabul-based analyst, Haroon Mir said: "This is the first Taliban spring offensive launched under their new leadership.

"Mansour has persistently rejected peace talks and insisted on war. Therefore he is expected to focus more on battlefield victories this year - that could mean a worse year for Afghanistan in terms of violence and bloodshed."

It has also prompted calls for the US to reconsider its troop withdrawal schedule, already delayed once by President Barack Obama.

There are currently 9,800 American troops in the country, with the number set to fall to 5,500 by 2017.

General John Nicholson, the commander of US and NATO forces in Afghanistan since March, had promised during his US Senate confirmation hearings to review the drawdown plan.

American forces are being increasingly drawn into fighting despite the official end of their combat mission, partnering with Afghan forces particularly in southern Helmand.

Last year, 22 US personnel were killed in Afghanistan, half of the deaths classified as "hostile", according to the icasualties.org website which tracks the war.