European security officials are scrambling to meet a changing and more complex threat from jihadists -- both from sleeper cells and fighters returning from Middle Eastern battlegrounds -- made clear in the deadly Paris attacks.

European police agency chief Rob Wainwright said the security landscape is "more difficult, more challenging" than at any time since the September 11, 2001 attacks.

It is an extremely dangerous time, stressed British Prime Minister David Cameron, as he spoke of a "severe" threat in which an attack is "highly likely."

The three days of violence that left 17 dead in Paris last week -- starting when gunmen stormed into the offices of satirical newspaper Charlie Hebdo January 7 -- have left the world reeling, with questions raised about how the perpetrators slipped through the cracks.

Some in Europe have called for tighter border controls and stricter immigration measures.

French far-right leader Marine Le Pen went further, calling for stripping "jihadists" of their French citizenship and urging Paris to denounce the attackers as "Islamists."

"Let us call things by their rightful names, since the French government seems reluctant to do so," she wrote in a New York Times op-ed.

"France, land of human rights and freedoms, was attacked on its own soil by a totalitarian ideology: Islamic fundamentalism."

- Working independently? -

The two Charlie Hebdo attackers, brothers Said and Cherif Kouachi, had ties to jihadist groups in Yemen and Syria.

Cherif and a third gunman, Amedy Coulibaly, who killed four hostages at a Jewish supermarket, had each spent time in jail where they were further radicalized.

But the three men had not been active in jihadist circles for nearly a decade before the Paris attacks, so police focused their attention elsewhere, Europol chief Wainwright said.

The challenge has shifted since Al-Qaeda's heyday under Osama Bin Laden, Wainwright stressed.

Police are seeing "a lot of independent or semi-independent people" who have been radicalized through the Internet or through experience fighting in Syria and Iraq, he told ABC's "This Week" in an interview that aired Sunday.

"Of course, that makes it much more dangerous. That's the challenge the police face," he said.

"It's much looser than we have seen before. It's not the same as in the days of 9/11, when we had an identifiable command and control structure."

US Senator Richard Burr said the Paris assault means authorities should re-evaluate how they monitor possible threats.

"Every country in the world today is probably looking back at the policies that they've got on surveillance for known fighters," the Republican lawmaker told CNN's "State of the Union."

- 'Perseverance' -

But Cameron spoke of a "very long" struggle against extremists.

"We'll have to show real perseverance," he said in an interview with CBS "Face the Nation" taped on Friday after his meeting with US President Barack Obama.

And the fight goes beyond just police and military action.

"We've also got to demonstrate that our values, that the things we stand for and care about in our societies -- of democracy and free speech and rights and the ability to have peaceful and progressive societies -- that those things are stronger" than the extremists' "poisonous death cult narrative," Cameron said.

"We cannot do this on our own as Western countries. We need functioning government in Iraq, functioning government in Syria, to be the legitimate authorities that with us, help to stand back this perversion of the Islamic religion.

"I think in a free society, there is a right to cause offense about someone's religion," Cameron added.

He noted that as a Christian, he might be offended by somebody's remarks about Jesus, "but in a free society I don't have a right to wreak my vengeance upon them."

Charlie Hebdo's new editor-in-chief, meanwhile, defended the caricatures in an interview with NBC's "Meet the Press."

"Every time we draw a cartoon of Mohammed, every time we draw a cartoon of prophets, every time we draw a cartoon of God, we defend freedom of religion," Gerard Biard said.

"If God becomes entangled in politics, then democracy is in danger."