Iran has long been accused of supporting Shia militants in Iraq
Iran remains the "most active state sponsor of terrorism" in the world, a report by the US state department says.
It says Iran's role in the planning and financing of terror-related activities in the Middle East and Afghanistan threatens efforts to promote peace.
Al-Qaeda remains the biggest danger to the US and the West, the annual report states, noting that terror attacks are rising in Pakistan.
Iran rejected the report, saying the US was guilty of double standards.
Foreign Minister Manouchehr Mottaki said the US had no right to accuse others in light of its actions at Abu Ghraib and Guantanamo Bay.
The BBC's state department correspondent, Kim Ghattas, says the new US administration may be trying to engage Tehran, but, just like last year, Iran is still described as the most active state sponsor of terrorism.
The report singles out the Quds unit of Iran's Revolutionary Guard
The report charges that Iran's involvement in countries like Lebanon, Iraq, Afghanistan and in the Palestinian territories threatens efforts to promote peace, economic stability in the Gulf and democracy.
The report singles out the Quds force, an elite branch of Iran's Revolutionary Guard as the channel through which Iran supports terrorist activities and groups abroad.
The report also takes to task Syria, an Iranian ally in the region.
Of equal concern, our correspondent notes, is the advance of al-Qaeda in Pakistan and Afghanistan where terrorist attacks are sharply on the rise while the rest of the world, including Iraq, has seen terrorist attacks decrease.
The acting coordinator for counter-terrorism for the state department, Ronald Schlicher, told journalists that al-Qaeda was using border areas of Pakistan to regroup.
"Al-Qaeda and al-Qaeda associated networks remain the greatest terrorist threat to the US and its partners," he said.
Mr Schlicher said they were using the Afghan-Pakistan border area "as a safe haven where they can hide, where they can train, where they can communicate with their followers, where they can plot attacks and where they can make plans to send fighters to support the insurgency in Afghanistan".
Washington is worried that the government in Islamabad might collapse, and last week US Secretary of State Hillary Clinton said the Taleban fighters posed an existential threat to Pakistan, which is a nuclear power, our correspondent adds.