Al-Qaeda is on a 'path of decline' following the deaths of top leaders but its affiliates are rising a threat in vulnerable regions, the US State Department warned in a report on Tuesday.

In its Country Reports on Terrorism 2011, the department branded Iran "the world's leading sponsor of terrorist activity" providing funds and support "for terrorist and militant groups throughout the Middle East."

Both Iran and Al-Qaeda are helping to foment unrest by spreading "violent extremist ideology and rhetoric" in some of the world's most restive regions, the report to Congress maintained.

Hailing the killing of Al-Qaeda leader Osama bin Laden in a US commando raid on his Pakistani compound in May 2011, the report noted he had "remained deeply involved in directing (the group's) operations and setting its strategy."

"The loss of bin Laden and these other key operatives puts the network on a path of decline that will be difficult to reverse," it said.

But, while the core group of Al-Qaeda may have been weakened over the past year, "we have seen the rise of affiliated groups around the world."

Al-Qaeda in the Arabian Peninsula was "a particularly serious threat" having gained territory in southern Yemen, and "was exploiting unrest in that country to advance plots against regional and Western interests," the report said.

The group's north African branch, known as AQIM, "historically the weakest of the affiliates, saw its coffers filled in 2011 with kidnapping ransoms -- a practice that other terrorist groups are also using to considerable advantage."

Al-Qaeda affiliates were also at work in Iraq, capitalizing on the withdrawal of US forces from there, despite suffering recent leadership losses.

The Iraqi branch was "resilient" and "believed to be extending its reach into Syria and seeking to exploit the popular uprising against the dictatorship of Bashar al-Assad."

The report also highlighted concerns about Boko Haram militants in Nigeria, said to have Al-Qaeda ties, and about Egypt's Sinai Peninsula where "a number of loosely knit militant groups have formed... with some claiming ties and allegiance to Al-Qaeda."

The other main terror threat to the United States remained Iran, which was designated by the US as a State Sponsor of Terrorism in 1984.

"Iran remained an active state sponsor of terrorism in 2011 and increased its terrorist-related activity," the report said.

The Islamic Republic's aim was "likely in an effort to exploit the uncertain political conditions resulting from the Arab Spring, as well as in response to perceived increasing external pressure on Tehran."

A plot uncovered in September to kill Saudi Arabia's ambassador to the US "underscored anew Iran's interest in using international terrorism -- including in the United States -- to further its foreign policy goals."

Tehran was also continuing to provide arms and training to militant groups such as Hamas and other Palestinian extremist groups, as well as Hezbollah.

And Iran's Revolutionary Guards had provided training to Taliban militants in Afghanistan.

The report also charged that Iran was allowing Al-Qaeda members to use its territory as a pipeline to funnel funds and operatives into South Asia.