WASHINGTON: Fifteen years after the September 11 attacks, US anti-terror officials say the country is hardened against such well-developed plots but remains as vulnerable as ever to small and especially home-grown attacks.

Counter-terror operations are under huge pressure to ferret out and disrupt plots by sympathisers of the militant Islamic State (IS) group and Al Qaeda hidden by less centralised networks and new communications technologies, they say.

“Our job is getting harder,” said Nick Rasmussen, the powerful director of the National Counterterrorism Center, at a stock-taking this week in Washington.

The explosion of ways extremists can communicate with each other, many of them via popular smartphone apps and easy access to powerful encryption, “gives them the edge” against the US intelligence community, he said.

The 9/11 attacks gave birth to the US War On Terror, which initially focused on Al Qaeda and the Taliban.

But 15 years later, the target is a different group, IS, which has seized territory in Syria and Iraq and shown the ability to plan and inspire home-grown attacks in Europe and the United States, smaller-scale than 9/11 but nevertheless deadly and demoralising.

Meanwhile, Al Qaeda still exists without former leader Osama bin Laden, with affiliates, spinoffs and rivals of both groups operating from the Philippines to West Africa, posing a more complex threat.

“The reality is that it has metastasized” from the Iraq-Syria region, said Frank Cilluffo, director of the Center for Cyber and Homeland Security at George Washington University.

“The threat persists and is in some cases more complex.”

A series of surprise attacks have placed “HVEs” — homegrown violent extremist — as much in the focus of intelligence agencies as threats from abroad.

Among them, a 29-year-old American of Afghan descent believed to hold radical sympathies shot dead 49 people in an Orlando gay nightclub in June.

And in December, a US-born man and his wife, both with Pakistani roots, killed 14 at a Christmas party in San Bernardino, California.

The George Washington University Program on Extremism counts 102 people who have been charged in the United States with offenses related to IS, many of them lured online.

US intelligence is strained by the more than 1,000 cases of possible extremists it is following, Rasmussen said. Moreover, plots are now developed and carried out much more rapidly, and in smaller networks, making it much harder for counter-terror operations to discover them.

US officials say they are confident IS will be defeated on its Iraq-Syria turf eventually, but that that won't end the overall extremist threat.

A breakup of IS could send hundreds of sympathisers underground around the world, lying quietly in wait for years to build new networks and plot attacks, they said.

“The threat that I believe will dominate the next five years for the FBI will be the impact of the crushing of the caliphate,” or the IS group, said James Comey, director of the Federal Bureau of Investigation.

That will release “hundreds of hardened killers” into the general population, many of them going north to hide in Europe, he predicted.

“We are facing this 'going dark' phenomenon where we cannot see these people,” he said.

The other big challenge, the officials said, is the weakness of European intelligence to identify and track threats, which they tied to still-weak cooperation between agencies in the different countries.

Rasmussen said that he had been more confident a decade ago in the ability of the US and other countries to work together in fighting terrorism.

Today, he said “I feel like we're pushing uphill,” and cooperation remains strongest on a bilateral basis.

The core fight is in ideology, officials also say, and the US has made little progress in combatting the propaganda that draws sympathisers to the IS group and Al Qaeda.

Real progress requires a longer-term strategy involving social media, said Michael Leiter of defence and intelligence contractor Leidos.

Only a little money is being given to people on the ground to fight radicalisation, he complained.

"There are no silver bullets here. Banning Muslims is not going to do it."