Iran has blasted arch-foe the United States over accusations Tehran is arming and training militants in Afghanistan, where US-led forces are battling a raging Taliban insurgency.

US Rear Admiral Gregory Smith charged on Monday that the Islamic republic was offering "support in training, financial support, and equipment, mostly ammunition" to Afghan militants.

Iran said his claims were unsubstantiated.

"Remarks by Americans accusing Iran of supporting militants in Afghanistan are baseless and unacceptable," foreign ministry spokesman Ramin Mehmanparast told English-language Press TV on Tuesday.

"Stability in Afghanistan is directly linked to stability in the region and in Iran. It is therefore quite natural for us to support every effort aimed at stabilising Afghanistan."

Mehmanparast said the White House was concerned about opposition to the US presence in the Middle East.

"What is worrying American officials is the wave of popular protests against the US presence in the region and its interference in their political and military affairs," he said.

Smith was forthright in his accusation against Tehran.

He said Iran was "giving them a limited amount of bullets, technical pieces of IEDs (improvised explosive devices), rockets, RPGs (rocket-propelled grenades), through networks well established through the border."

Smith, deputy chief of staff of ISAF, the International Security Assistance Force in Afghanistan, said of Iran: "They want to be in the game."

There are 140,000 foreign troops, most of them American, deployed in Afghanistan to help defeat the Taliban, who have waged an increasingly bloody insurgency since they were ousted from power in a US-led invasion in 2001.

"They are building networks," Smith said of Iran.

ISAF was monitoring Iranian activity carefully, he added.

"We are watching what in the longer term Iran is up to. Destabilising Afghanistan is not good for Iran," Smith said.

The NATO-led ISAF force is due to start limited withdrawals in safer parts of Afghanistan in July, a process which could start in the west of the country, on the border with Iran.