"There's irrefutable evidence the Iranians are now doing this," Undersecretary of State Nicholas Burns said 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 said NATO must 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 U.N. Security Council resolutions.

The State Department later appeared to step back from Burns' assertion the Iranian government was directly involved in the transfers but stressed Washington has proof that weapons from Iran were being sent to Taliban fighters in Afghanistan.

"We absolutely are certain that there are Iranian-origin weapons flowing into Afghanistan to the Taliban," spokesman Sean McCormack told reporters.

"We do not know the extent of any Iranian government involvement at this point, but given the nature of the regime and also some of its past behaviors elsehwhere — whether in the Palestinian areas or in Iraq — it certainly raises very serious questions and we are quite concerned about it," he said.

Tehran, which is also in a dispute with the West over its nuclear program, denies it is aiding the Taliban, calling the accusation part of a broad anti-Iranian campaign. Iran says it makes no sense that a Shiite-led government like itself would help the fundamentalist Sunni movement of the Taliban.

Burns acknowledged that it was "curious" that Iran would aid the Taliban.

"It's quite surprising," he told CNN. "The Iranians had said that they were the mortal enemies of the Taliban in 2001 and '02."

Burns did not give details on the scope of the alleged Iranian shipments, although he appeared to indicate that they were limited. "I don't think it's made a substantial difference in the greater theater of the war," he said.

"It is not going to turn the tide against us, but it is very troublesome, it is illegal under international law ... and the Iranians need to stop it," Burns told the AP.

Burns, who was holding talks in Paris, first accused Iran on Tuesday of transferring weapons to the Taliban insurgency in Afghanistan — the most direct comments yet on the issue by a ranking American official.

In Afghanistan last week, Defense Secretary Robert Gates said Iranian weapons were falling into the hands of anti-government Taliban fighters, but he stopped short of blaming Tehran.

Iran's possible role in aiding insurgents in Iraq has been hotly debated, and last month some Western and Persian Gulf governments alleged that the Islamic government in Tehran is also secretly bolstering Taliban fighters.

In an AP interview Monday, U.S. Army Gen. Dan McNeill said Taliban fighters are showing signs of better training, using combat techniques comparable to "an advanced Western military" in ambushes of U.S. Special Forces soldiers.

"In Afghanistan it is clear that the Taliban is receiving support, including arms from ... elements of the Iranian regime," British Prime Minister Tony Blair wrote in the May 31 edition of the Economist.

On the Iranian nuclear issue, Burns claimed that sanctions already leveled against Tehran were being felt and reiterated the threat of more if the country refuses to suspend uranium enrichment — which the West fears could be meant for the production of nuclear weapons.

President Mahmoud Ahmadinejad on Wednesday dismissed the possibility that a third set of Security Council sanctions would harm Iran.

Burns disagreed. "I think most people would say that the Iranians are experiencing considerable economic difficulties because of the financial sanctions that have been taken outside the Council and because of Security Council sanctions," he told CNN.