Nato special forces have intercepted a large shipment of Iranian-made arms destined for Taliban insurgents fighting British troops in Afghanistan's Helmand province.

The haul included a number of deadly "explosively-formed projectiles" used in roadside ambushes and capable of penetrating even tank armour.

The shipment is the third seized this year. Two smaller convoys carrying weapons from Iran were seized in Helmand in April and May.

A Nato source said yesterday that "Western special forces" had interdicted the arms in Farah province, a vast desert area which is sparsely inhabited and poorly policed.

He added: "The Taliban are obviously trying to vary their routes after losing their last two supply convoys. This interception was intelligence-led.

"It was size of the shipment that has grabbed our undivided attention this time round. The insurgents were obviously planning a major offensive at some point or trying to stock up in a single throw for a longer campaign."

The Taliban have lost an estimated 3000 fighters in battles with British, Dutch, Canadian and US troops this year.

Nato sources said that, while the hardline insurgents were still prepared to tackle allied forces in conventional ambushes, it was clear that the organisation's leaders were shifting the emphasis to more indirect action.

While Britain and the Afghan government have played down accusations that the Iranian government is directly involved in smuggling arms across the border, the US has publicly accused Tehran of complicity in terrorism in both Iraq and Afghanistan.

Military sources say the likely sources of arms are Iran's Revolutionary Guard Corps, a 120,000-strong organisation which answers to the country's ruling clerics rather than its high command, or powerful drug smugglers who want to prevent Nato from interfering with their opium supplies.