Concern is rising in Europe that civil war in Syria and foreign military action against Islamist rebels in Mali have given new impetus to the radicalisation of disaffected young Muslims.
France, in particular, fears scores of its own citizens, most of North African or sub-Saharan origin, are either potential recruits for Al Qaeda-linked groups directly involved in the insurgencies or capable of mounting attacks on French soil.
There have been arrests in Paris this month of men alleged to be intent on fighting French and African forces in northern Mali and, in recent weeks, the deportation of non-French imams accused of preaching extremism.
But security forces are also on the alert in the Netherlands and Scandinavia. In November, police in the Dutch port of Rotterdam arrested three Muslims who, according to prosecutors, wanted to join militant Islamists in Syria.
Norwegian, Swedish and Danish officials, too, have raised concerns about attempts to lure their nationals to the Syrian uprising.
The alarm reflects the growing European and North American view that while the regime of the Syrian president, Bashar Al Assad, is unsustainable, some of the forces ranged against him are profoundly anti-western.
The French interior minister, Manuel Valls, said last week about 100 people, by implication Muslims, of French nationality or residence had gone to Syria to join anti-government combat. Of these, a "handful" had also been active in the Sahel region of Africa, including northern Mali, and in Somalia.
Mr Valls, interviewed by the daily newspaper, Le Parisien, said: "Our police and intelligence services must constantly harass these groups."
He added that he had expelled from France "without hesitation" non-nationals using mosques or the internet to attack "the fundamental interests of our country".
"We face an external enemy in Mali, we are also facing an enemy within resulting from a process of radicalisation," Mr Valls. "It starts with petty crime through drug trafficking, sometimes in prison, and leads towards conversion to radical Islam and hatred of the West."
Mr Valls recalled the case of Mohamed Merah, the French-Algerian petty criminal who a year ago murdered two French soldiers and four Jews, including three children and the father of two of them. Merah claimed before being killed in a shoot-out with police that he had acted on behalf of Al Qaeda.
There were still "several tens of potential Merahs" in France, the minister said, even if they had not put their thoughts into action. "In March 2012, Mohamed Merah killed soldiers because they were soldiers, killed Jewish children and a father because they were Jews."
French authorities, he said, had dismantled networks that had struck, or were preparing to strike, in ways he did not specify.
Of the four people, aged between 22 and 37, arrested earlier this month in or near the Parisian suburb of L'Hay les Roses, three are French nationals — one holding joint Franco-Algerian citizenship — and other is a Malian, according to a judicial source.
Officials believe they are linked to a man identified as Cedric Lobo, detained in August on the Niger-Mali border as he attempted to join Islamist fighters in northern Mali. He was sent to France where he is under investigation for criminal association with plans to prepare a terrorist act.
Last November, Malian authorities arrested Ibrahim Ouattara, 24, a French-Malian from another Parisian suburb, Aubervilliers. He had allegedly made previous visits Yemen and Somalia and is suspected of scouting for a planned recruiting network for the Malian conflict.
Although full details of the hostage-takers who attacked the In Amenas gas plant in Algerian in January remain unclear, Mr Valls said during the crisis at least one was French. A Canadian was also said to have played a prominent role.
The extent of alienation of many Muslim youths on poor, immigrant-dominated estates or neighbourhoods is a familiar feature of radicalisation in France.
In a separate interview, on French television after the assassination in Tunisia of the opposition politician Chokri Belaid, Mr Valls, seen as a rising star in France's struggling socialist government, attacked "a rising Islamic fascism everywhere.
He said "this darkness must obviously be condemned, since it negates the rule of law, the democracy for which the people Libya, Tunisia, Egypt fought".
Meanwhile, moderate Muslim leaders across Europe are worried that political leaders may be playing into the hands of right-wing extremists by stigmatising followers of the faith who merely wish to live in peace.
But an iman from Drancy, near Paris, where Jews were held in the Second World War before being deported to Nazi death camps, believes moderates must take the lead. He has co-written, with a political journalist, David Pujadas, a book entitled Let Us Act Before It Is Too Late.
Hassen Chalghoumi, whose views - opposed to face-covering veils, in favour of dialogue with Jews - divide Muslims and have even brought him death threats, urges vigilance to prevent the spread of hatred.
He writes: ""Before, there were groups, more or less organised. You could keep an eye on them. Nowadays, it is individuals, young people, we do not always see going astray.
"Everyone has be vigilant, especially parents … Parents can be proud of their children having faith, but we must always be aware of the state of mind of the child, listen to his words, his manner of speaking, his way of reacting and to know which websites he consults."
* Additional reporting by the Associated Press