Terrorist attacks worldwide surged by more than a third and fatalities soared by 81 per cent in 2014, a year that also saw Islamic State eclipse al Qaeda as the leading jihadist militant group, the US State Department said on Friday.

In its annual report on terrorism, the department also charts an unprecedented flow of foreign fighters to Syria, often lured by Islamic State's use of social media and drawn from diverse social backgrounds.

Taken together, the trends point to a sobering challenge from militant groups worldwide to the United States and its allies despite severe blows inflicted on al Qaeda, author of the Sept. 11, 2001 attacks in Washington and New York.

Al Qaeda's leaders "appeared to lose momentum as the self-styled leader of a global movement in the face of ISIL's rapid expansion and proclamation of a Caliphate," the report said, using an alternate acronym for Islamic State.

Last June, Islamic State attacked from its base in Syria and seized vast swaths of Iraq, much of which it still controls.

"The ongoing civil war in Syria has been a spur to the worldwide terrorism events," the State Department's coordinator for counterterrorism, Tina Kaidanow, told a news conference.

She said that while the United States still worried about al Qaeda, the growing concern was the number of groups aligning themselves with Islamic State across the globe.

"There is an appeal of ISIS globally," Kaidanow added, using another acronym for the group.

US President Barack Obama responded with air strikes in Iraq and Syria, and a program to train Iraqi security forces. He has also continued air strikes against militant suspects worldwide, included one this week that killed al Qaeda's deputy chief.

The State Department report, which covers calendar year 2014, said there were 13,463 terrorist attacks, a 35 per cent jump from 2013, resulting in more than 32,700 deaths, an 81 per cent rise. More than 9,400 people were kidnapped or taken hostage by militants, triple the rate of the previous year, it said.

There was some good news: Militant activity decreased in some countries, including Pakistan, the Philippines, Nepal and Russia.

The report said the global increase in terrorist attacks was mostly due to events in Iraq, Afghanistan and Nigeria.

Kaidanow said weak or failed governments allowed terrorist groups to thrive in places such as Yemen, Syria, Libya, Nigeria and Iraq.

Islamic State was particularly lethal. A June 2014 attack on a prison in Mosul, Iraq, in which the group killed 670 Shi'ite Muslim prisoners "was the deadliest attack worldwide since September 11, 2001," it said.

As of late December, more than 16,000 foreign terrorist fighters had traveled to Syria, exceeding the rate of those who traveled to Afghanistan and Pakistan, Iraq, Yemen or Somalia "at any point in the last 20 years," the report said.

Last month, a senior State Department official said the army of foreign fighters who traveled to Syria had grown further, to 22,000. – Reuters