The US accused Iran of providing aid to the Palestinian group Hamas, the Lebanese movement Hezbollah, "Iraq-based militants", and Taliban fighters in Afghanistan.
The report said "elements" of Iran's Islamic Revolutionary Guard Corps were directly involved in the planning and support of terrorist acts throughout the Middle East.
The US said attacks in Iraq dropped slightly between 2006 and 2007 but they accounted for 60 per cent of worldwide terrorism fatalities.
More than 22,000 people were killed in attacks around the world in 2007 - eight per cent more than in 2006, although the number of actual attacks fell, the report said.
The US said al-Qaeda remained the greatest threat to its security despite ongoing efforts to combat the group.
"It has reconstituted some of its pre-9/11 operational capabilities through the exploitation of Pakistan's Federally Administered Tribal Areas, replacement of captured or killed operational lieutenants, and the restoration of some central control by its top leadership, in particular Ayman al-Zawahiri," it said.
The main reason for its resurgence was a ceasefire the Pakistani government reached with tribal leaders in the region last year, the report claimed.
That truce has since ended but Pakistan's new government is now negotiating a similar agreement that some in the US fear could further undermine efforts to battle al-Qaeda.
The earlier ceasefire and instability appear "to have provided al-Qaeda leadership greater mobility and ability to conduct training and operational planning, particularly that targeting Western Europe and the United States", the report said.
The report, published annually, gives an overview of what the US calls terrorist attacks worldwide.