Channel 4 News reveals Taliban insurgents fighting British and US forces are being supported by Iranian weapons - mines, mortars and plastic explosives - smuggled over the border, writes Nima Elbagir.
Please wait while this video loads. If it doesn't load after a few seconds you may need to have Adobe Flash installed.
The exclusive images and documents show, for the first time, the full extent of Iranian support for the Taliban in the shape of tonnes of weapons of the type being used against UK troops in Helmand province.
Despite the millions of dollars being spent by the international community to ensure cross-border security between Iran and Afghanistan, Channel 4 News has been shown vast hauls of weaponry which Afghan security services have told us are just a fraction of hardware intercepted from Iran on their way to the Taliban.
They claim it shows the true extent of direct support from the Iranian government for the insurgency.
The Afghan border with Iran is almost 1000km long and is incredibly difficult to police. The border town of Eslam Ghalah, in the Western Afghanistan province of Herat, is a key checkpoint for goods and human traffic entering and leaving Afghanistan.
Yet less than 10km from this border town is one of the country's most notorious smuggling routes. And Channel 4 News has learnt that it is also a major pipeline for Iranian support to the Taliban.
The range of weaponry shown to Channel 4 News includes mortars, plastic explosive, hand grenades, phone cards and Taliban propaganda booklets. There are also rows upon rows of mines - each with a Persian serial number. Proof, say the Afghan authorities, that they came from Iran's state run weapons factories.
The Afghan authorities seem to be finding it harder and harder to keep a lid on their increasing frustration. Rahmutallah Safi, the head of border police for Herat, invited local journalists to view their most recent haul.
"In this place you can see, we have discovered five mines," he said. "All the international monitors have seen it. You yourselves can check to see which country has made it. You can see the [Persian] marks on the weapons and the type and show it to the world.
"According to the Afghan government's own internal records, seen by Channel 4 News, over the last Islamic year 10.5 tonnes of weaponry was intercepted in Herat province alone - 60 per cent of which they say comes directly from the Iranian government.
Just in the last week authorities here claim to have intercepted over a tonne and a half of weaponry- and there is no way of knowing exactly how many smugglers are slipping through the authorities' nets.
But it is not just the Afghan authorities who acknowledge that there is Iranian support for the Taliban.
Nato's International Security Assistance Force (ISAF), which operates in Herat, said that they did believe there was "limited" Iranian support for the Taliban through weapons and training, however they did not believe that it was at a level that was "decisive" to the outcome of the anti-insurgency effort.
But, in a revealing interview with Channel 4 News, the Taliban have told us that Iranian backing is a major factor in their war against Nato tro
In the Western border to Kunduz in the North East of Afghanistan a senior member of the Taliban gave us an interview. Commander "Noori" - who would only talk on condition of anonymity - told us that Iran's importance to the Taliban was crucial especially as the West was pressuring Pakistan to close its borders to the Taliban.
"Day by day the Iranian border becomes more important for us," he said. "Especially now in Pakistan there are many problems for the Taliban and many of the Taliban have been imprisoned and also they arrest any Taliban who comes out of the Madrasas [religious schools]."
He told us it was not just the Iranian government's support that was crucial but also the continued ease of movement the Taliban were afforded over its border.
"The Mujaheddin themselves bring the weapons and money in [over the border] although we do also use professional smugglers to bring in our shipments."
And the United States seems finally to be waking up to the importance of Afghanistan's Western border. A new US consulate is due to open in Herat soon. It is the closest US Consulate to Iran but even before its new residents have moved in - already it has been hit by rocket attacks.
But back in the Afghanistan capital, Kabul, we met with a Western Official who gave us his take on the situation we had witnessed in the western province. He said Iran was "trying not to tip the balance.
They judge dominance by neither side is in their interest". In other words - Iran will provide just enough support to fuel the ongoing insurgency, bogging down the West, the Afghan government and the Taliban in this costly conflict.
"The Mujaheddin themselves bring the weapons and money in although we do also use professional smugglers to bring in our shipments." Commander "Noori"
In a statement to Channel 4 News the Iranian Embassy in London said: "The implementation and maintaining of security and peace in Afghanistan has always been stressed by the Islamic Republic of Iran as it has an impact on the security and peace in Iran too.
"Iran was the first country to recognize Afghanistan and there is ...close cooperation and friendship between the two countries. The Taliban are enemies of Iran and have killed a number of Iranian diplomats in Afghanistan.
"These allegations are fabricated to pervert attentions from the problems and damage created by foreign forces in that country."