IS a threat to West?

US President Barack Obama said on Wednesday during a visit to Estonia that his country's clear objective was to "degrade and destroy" the Islamist terror group "Islamic State" (IS).

His Vice President, Joe Biden, chose stronger words: The United States would pursue the killers of the two slain American journalist "to the gates of hell."

Some observers, however, fear IS fighters from Europe and the United States could easily return to their home countries with their Western passports and carry out terrorist attacks there.

More than 12,000 foreign fighters, including about 100 Americans and more than 1,000 Europeans, have traveled to Syria in the last three years, according to Matthew Olsen, director of the US National Counterterrorism Center. Terrorists with Western citizenship carrying out terrorist attacks in their homeland, Olsen said, is the "model that we are most concerned about."

IS more dangerous than al Qaeda?

For Olsen, there is "no group as effective and as successful using propaganda" as the self-proclaimed Islamic State. The Islamists reach many potential fighters via social media channels, including in the West.

"They are very adept at targeting a young audience," John Horgan, a psychologist at the University of Massachusetts, told US daily The New York Times. The Islamists lure recruits with this message: "Be part of something that's bigger than yourself, and be part of it now."

Olsen said the IS threatens to become the world's most dangerous terrorist organization soon. The group could become an even greater threat than al Qaeda, which was responsible for the September 11, 2001 terrorist attacks in the United States. The IS is well equipped financially: currently, the group takes in more than a million dollars a day, from sources including ransoms for hostages.

Covering their tracks

However, in contrast to al Qaeda, IS fighters are using different tactics, Olsen said. IS focuses less on large single attacks, but rather many small ones, such as the attack on a Jewish museum in Brussels. IS fighters also largely avoid electronic communication, which makes monitoring them more difficult.

But Olsen said there is currently no acute danger to the West.

"ISIL is not al Qaeda pre-9/11," he said, employing a previous name claimed by IS. US intelligence agencies are now much more aware of the issue of terrorism and are exchanging more information. There is currently no evidence of terrorist cells in the United States or Europe.

The fight against IS is still a long-term task, Olsen said. This must be dealt with politically, such as by forming a unity government in Syria to replace the Assad regime. Only then could the United States - as Obama announced - "degrade and destroy" the IS.