A plot reportedly originating in Pakistan to stage multiple raids in Europe shows the continent must do more to impede extremists going overseas to train and plan attacks, a counter-terrorism official said on Friday.
Gilles de Kerchove, the European Union counter-terrorism coordinator, said the conspiracy reported by intelligence sources this week should remind Europe of the need for vigilance five years after the continent's last militant attack.
"There's a number, a not insignificant number, of seriously dangerous people going around," he told Reuters.
"For some time there has been serious concern about people born or resident in Europe travelling to jihadi conflict zones - Afghanistan-Pakistan, Somalia, Yemen."
"This now confirms the importance of working on impeding the travel of jihadists and mobilising all the policies available to stop this phenomenon," he said in a telephone interview.
Western intelligence sources said this week militants in hideouts in northwest Pakistan were plotting coordinated attacks on European cities, the plans apparently surviving setbacks from a recent surge in drone strikes and an arrest.
The plot involved al-Qaeda and allied militants, possibly including European citizens or residents, the sources said.
Speaking on condition of anonymity, one security official in Germany said first word of the plot had probably come from the interrogation of a German-Afghan suspect in Afghanistan.
Kerchove, whose job focuses on policy and not operations, declined to discuss the details of the reports and would only say disclosures of any threat were "taken seriously".
He noted senior security officials in Germany, Britain and France had all spoken publicly in September about a resilient and evolving threat from extremist militants.
Such statements, and the recently disclosed plot, showed Europe had to remain alert, Kerchove said.
"They're not going public just for show. It's because it's serious.
"It confirms we have to remain extremely focused and concerned because 'no attack' does not mean 'no threat'. The security agencies can become victims of their own success."
The last successful large-scale militant attack in the West was the 2005 bombings on London's transport system which killed 52 people.
Joerg Ziercke, the head of Germany's BKA Federal Crime Office, said the attack threat was growing as the numbers of people coming back from militant camps on the Afghan-Pakistan border rose.
French interior minister Brice Hortefeux said France faced a real threat from al Qaeda militants in North Africa.
In Britain, Jonathan Evans, head of the MI5 security service, said there was a serious risk of a lethal attack taking place and militants in Yemen and Somalia were growing threats.
Security experts say monitoring violent Islamists in the West has been made more difficult by the rise of so-called self-radicalised militants with no previous record of extremism.
Al-Qaeda prizes such "home-grown" recruits as they have Western passports and can travel overseas easily, they say.
In a report to EU member governments in June Kerchove said al Qaeda remained the EU's biggest threat and urged better control on the movements of potential terrorists in the EU.
Al-Qaeda and South Asian allies have threatened to attack Western targets to avenge US military action in Afghanistan.
The Pakistan Taliban, the Pakistani militant group most influenced by al Qaeda, on September 3 threatened to launch attacks in the United States and Europe "very soon".