British Special Forces have intercepted a convoy of Iranian weapons destined for Taleban insurgents in southern Afghanistan.
Commanders fear the rockets - smuggled across the border on three pick-up trucks - represent a stark escalation in Tehran's support for the insurgents, at a time when record levels of United States and Nato forces are struggling to reverse a rising tide of Taleban violence.
General David Petraeus, the commander of US and Nato forces in Afghanistan, briefed Afghanistan's president Hamid Karzai on details of the haul yesterday, which included 48 122mm rockets and about 1,000 rounds of Kalashnikov ammunition.
One truck exploded before its cargo could be recovered, an official said.
The rockets, which are thought to have been destined for the Taleban's Spring Offensive in Helmand, have a range of 20km and a blast radius of about 80ft, roughly twice that of the ubiquitous 107mm rockets already used by the Taleban and usually fired at Nato bases.
"The 122mm rocket is a more advanced weapon, with a bigger blast area. It produces more shrapnel so it has the potential to produce more casualties," said an official at Nato's International Security Assistance Force.
"We have long been clear that there is some support for the insurgency from Iran. That has taken the form of small arms, ammunition, rocket-propelled grenades and 107mm rockets. This is the first instance where we have seen Iranian-produced 122mm rockets."
In a related development, the Nato officials said a Taleban leader travelled to Tehran recently and is believed to have asked the Iranians to provide more shoulder-fired anti-aircraft missile systems, such as the two Iran provided in 2007 which were used against one British and one US Chinook helicopter, the official said
Mr Karzai's spokesman, Waheed Omer, said Afghan officials would launch their own investigation to ascertain whether the Nato allegations were true. Mr Omer said that would include "talking to our Iranian friends".
"The president was briefed by (Gen Petraeus] about it, but of course we need to do our own investigation," Mr Omer said. "We hope that this is not the case, because we have had excellent relations with Iran over the last few years and we would like this relationship to continue on a friendly basis."
Details of the cargo emerged as the United Nations announced 2,777 civilians were killed in the conflict in 2010, a 15 per cent increase on 2009.
It said the Taleban were responsible for the "vast majority".
Western officials said the rockets, seized during an SAS raid on 5 February, were the clearest proof yet of Tehran's duplicity.Last year Mr Karzai admitted his chief of staff, Omar Daoudzai, regularly accepted plastic bags full of cash from Iranian officials as part of normal neighbourly relations.
Although the serial numbers on the rockets had been doctored to try to disguise their origin, forensic tests proved that they originated in Iran.
The convoy was intercepted close to the Iranian border, in Nimroz province, after getting stuck in soft mud.
Three smugglers escorting the weapons were killed in the raid.