Iran is "the most active state sponsor of terrorism", according to the US state department's annual report on world terrorism.
Iran has made clear its support for suicide bombings in Israel
It finds that Iran's Revolutionary Guards and intelligence ministry are directly involved in planning and supporting terrorist acts.
There was no immediate response from Tehran which is locked in a row with the US over its nuclear programme.
The report also argues that al-Qaeda had been weakened.
The US report says other state sponsors of terrorism include Libya, North Korea, Sudan, Syria and Cuba.
Cuba, for example, is accused of harbouring members of Spain's Eta militant group and Colombian leftist rebel groups.
Iraq, the report adds, is "not currently a terrorist safe haven" although Islamist militant groups view it "as a potential safe haven and are attempting to make it a reality".
BBC State Department correspondent Jonathan Beale says that, by highlighting successes in disrupting the al-Qaeda network, the US hopes to convey the message that it is winning the war on terror.
But the dramatic rise in the number of terrorist attacks paints a very different picture, our correspondent adds, even though the US puts that down to a new broader definition of what is a terrorist attack.
Other findings include:
- Last year saw 11,000 terror attacks worldwide and 14,600 deaths, 8,300 of them in Iraq
- Suicide attacks increased in several countries and caused more than a fifth of all terror deaths worldwide
- The number of terror victims in Israel was 50 - half the total calculated by the US for 2004
- Safe havens for terrorists include the Afghan border, the Celebes Sea in South-East Asia, Somalia; and the intersection of Argentina, Paraguay and Brazil
'Years of war ahead'
"The international community's actions have degraded the core al-Qaeda leadership group," the report says.
"Its core leadership no longer has global command of its network."
There is evidence, the report continues, that core leaders such as Osama Bin Laden and Ayman al-Zawahri are "somewhat frustrated by their lack of direct control".
However, the report also argues that current terror groups are looser networks and therefore could pose greater danger.
America, it says, is in "the first phase of a potentially long war".
"The enemy's proven ability to adapt means we will go through several more cycles of action/reaction before the war's outcome is no longer in doubt," the US report says.
"It is likely we will have a resilient enemy for years to come."