Speaking at a joint press conference with Afghan President Hamid Karzai in Kabul today, Gates said he does not know how the arms reached Afghanistan but the matter will be monitored.
"There have been indications over the past few months of weapons coming in from Iran," Gates said. "We do not have any information about whether the government of Iran is supporting this, is behind it, or whether it is smuggling, or exactly what is behind it. But there clearly is evidence that some weapons are coming into Afghanistan -- destined for the Taliban, but perhaps also for criminal elements involved in the drug trafficking -- coming from Iran. We are watching it very carefully, and we will be staying in very close touch about it."
In April, top U.S. military officer General Peter Pace said Iranian-made mortars and explosives heading to the Taliban had been intercepted in Kandahar Province.
Gates today also blamed the Taliban for most of the civilian casualties during military operations in Afghanistan, saying the insurgents usually hide among civilians.
Gates said earlier predictions about a large Taliban spring offensive have not materialized. He said the Taliban will be "defeated soon."