The United States has "irrefutable" evidence that Iran is transferring weapons to the Taliban in Afghanistan, with the knowledge of the Iranian government, and NATO has intercepted some of the shipments, a senior U.S. diplomat said Wednesday.

"There's irrefutable evidence the Iranians are now doing this," said Undersecretary of State Nicholas Burns on CNN. "It's certainly coming from the government of Iran. It's coming from the Iranian Revolutionary Guard corps command, which is a basic unit of the Iranian government."

Speaking separately to The Associated Press, Burns also said that NATO needs to act to stop the shipments. The Iran-Afghanistan frontier is "a very long border. But the Iranians need to know that we are there and that we're going to oppose this."

"It's a very serious question," he said, adding that Iran is in "outright violation" of relevant U.N. Security Council resolutions.