Elements in the Iranian state are sending weapons across the border to the Taleban in Afghanistan, a BBC investigation has uncovered.
Taleban members said they had received Iranian-made arms from elements in the Iranian state and from smugglers.
The UK says its troops have intercepted arms which it believes were given by a group within the Iranian state.
The Iranian embassy in Kabul dismissed the allegations, saying Tehran supported the Afghan government.
'It's called Dragon'
Among the Taleban, Iranian-made weapons are greatly sought after in the fight against the government of Hamid Karzai and the Nato and US-led forces deployed to support him.
They are considered to be reliable and particularly destructive.
For example, an Iranian-made Kalashnikov rifle can be adapted to fire grenades. It costs $200-$300 more than one made in Pakistan, Russia or China.
Iranian-made weapons, one commander told me, had really improved the Taleban's ability to attack the American military deployed in his area.
"There's a kind of mine called Dragon. Iran is sending it, we have got it," the Taleban commander said.
"It's directional, it destroys. If you lay an ordinary mine, it will cause only minor damage to Humvees or one of their big tanks. But if you lay a Dragon, it will be destroy it completely."
The question is - are Iranian weapons only being brought across the border by smugglers for profit or are elements of the Iranian state also donating arms?
The Taleban commander and other sources in the south told me both routes were operating.
The British ambassador in Kabul, Sir Sherard Cowper-Coles, made the same allegation.
"We've seen a limited supply of weapons by a group within the Iranian state, not necessarily with the knowledge of all other agencies of the Iranian state, sending some very dangerous weapons to the Taleban in the south.
"It's a very dangerous game for Iran, a Shia state, to be supplying Sunni extremists, like the Taleban."
Seven years on, the worsening of American-Iranian relations might give some groups within the Iranian state a reason to help the Taleban fight their mutual enemy - the US army deployed in Afghanistan.
Kate Clark's full report will be broadcast on Newsnight on BBC2 on 15 September at 2130 GMT, and on Assignment on the BBC World Service on 18 September. http://news.bbc.co.uk/1/hi/world/south_asia/7616429.stm